materials

Article
Corrosion Behavior of Pressure Infiltration
Diamond/Cu Composites in Neutral Salt Spray
Zhongnan Xie 1,2,3 , Hong Guo 1,2,3, *, Ximin Zhang 1,2,3 and Shuhui Huang 1,2,3
1 State Key Laboratory of Nonferrous Metals and Processes, GRINM Group Co., Ltd., Beijing 101400, China;
zhongnanx@126.com (Z.X.); zxmbeibei@126.com (X.Z.); hithuang@126.com (S.H.)
2 GRIMAT Engineering Institute Co., Ltd., Beijing 101400, China
3 General Research Institute for Nonferrous Metals, Beijing 100088, China
* Correspondence: guohong@grinm.com; Tel.: +86-1360-121-6895




Received: 15 March 2020; Accepted: 7 April 2020; Published: 14 April 2020


Abstract: Diamond particle-reinforced copper matrix composites (Diamond/Cu) are recognized as

promising electronic packaging materials due to their excellent thermophysical properties. It is
necessary to investigate the reliability of Diamond/Cu composites under extreme environmental
conditions. The corrosion behavior of Diamond/Cu composites was studied in a 5 wt% NaCl neutral
salt spray. Surface morphology, thermal conductivity, bending strength, corrosion rate, and corrosion
depth resulting from corrosion were researched in this paper. The results showed that the corrosion
phenomenon mainly occurs on the copper matrix, and the diamond and interface products do not
corrode. The corrosion mechanism of Diamond/Cu composites was micro-galvanic corrosion. The
corrosion product formed was Cu2 Cl(OH)3 . The salt spray environment had a great influence on the
composite surface, but the composite properties were not significantly degenerated. After a 168-h
test, the bending strength was unaltered and the thermal conductivity of gold-plated composites
showed a slight decrease of 1–2%. Surface gold plating can effectively improve the surface state and
thermal conductivity of Diamond/Cu composites in a salt spray environment.

Keywords: metal matrix composites; Diamond/Cu composites; neutral salt spray; corrosion behavior;
thermal conductivity

1. Introduction
Due to their high thermal conductivity and adjustable thermal expansion coefficient, Diamond/Cu
composites have great potential for heat sinks and base plates in high performance electronic
packaging [1,2]. Compared with current commercial thermal management materials, such as W-Cu [3],
Mo-Cu [4], SiC/Al [5], etc., Diamond/Cu composites have outstanding performance advantages in
thermal conductivity (>600 W/mK) [6,7]. Electronic packaging materials should possess not only high
thermal conductivity, but also reliability and stability of performance. It is necessary to investigate
the reliability and performance evolution of Diamond/Cu composites under extreme environmental
conditions before further application.
Metal matrix composites (MMCs) are usually composed of metal matrices and reinforcements
with different physical and chemical properties. In a neutral salt spray environment, the metal matrix
and reinforcement exhibit different electrochemical corrosion potentials and corrosion characteristics.
The corrosion mechanism of MMCs is mainly in the following forms [8–10]. Compared with monolithic
matrix alloys, the introduction of the reinforcement phase changes the homogeneity of structure and
composition, making the matrix more susceptible to localized corrosion. Interfacial reaction products,
impurities, defects, etc., all affect the corrosion resistance of the composites. The corrosion behavior of
MMCs is also affected by the formation of interfacial products in the process of fabrication [11,12]. Due

Materials 2020, 13, 1847; doi:10.3390/ma13081847 www.mdpi.com/journal/materials

Materials 2020, 13, 1847 2 of 11

to the mismatch of the thermal expansion coefficient between the reinforcement and matrix, there will
be stress and high dislocation density at the interface, which will accelerate corrosion [8].
Corrosion behavior of copper matrix composites reinforced with diamond, SiC, graphite, and
graphene has been reported [13–17]. Due to the difference of the reinforcement phase, the corrosion
behavior of composite materials is different. In the 3D-SiC-reinforced copper matrix composites, copper
corrosion at the matrix interface is severe. This position is liable to form a corroded galvanic cell
because of uneven chemical properties and high residual stress. Graphite/Cu composites have better
corrosion resistance than copper. Corrosion occurs at grain boundaries, rather than at the interface
between the graphite and copper matrix [16]. However, some scholars believe that graphite has a
more noble potential than copper, and the galvanic coupling in the sample leads to the increase of the
local corrosion rate. The corrosion resistance of Graphene/Cu composites is related to the arrangement
of graphene. Transversely arranged graphene can greatly improve the corrosion resistance of the
composite [15]. In addition to the types of reinforcements, the corrosion behavior of composites is
also affected by the reinforcement content. It has been found that the corrosion rate of the composites
decreases with the increase of the content of reinforcing materials [18,19].
A great deal of work has been done on the thermophysical and mechanical properties of
Diamond/Cu composites. However, systematic work on the corrosion behavior and property evolution
of Diamond/Cu composites has not yet been carried out. We used 60 vol% and 75 vol% Diamond/Cu
composites to carry out salt spray tests, mainly because these two types of composites have excellent
comprehensive properties, such as high thermal conductivity, outstanding mechanical properties,
and a semiconductor-matched thermal expansion coefficient. They are the two types of Diamond/Cu
composites with the greatest potential for large-scale application in the future. In view of this, the
purpose of this study was to investigate the effect of corrosion on the surface condition, microstructure,
and properties of Diamond/Cu composites during a neutral salt spray test. Based on analyzing the
corrosion rule of Diamond/Cu composites, feasible anticorrosion improvement is put forward in order
to provide guidance for the future application of this type of material.

2. Materials and Methods

2.1. Materials
Diamond/Cu composites were prepared by pressure infiltration. The diamond sizes in 60 vol%
Diamond/Cu composites were 100 µm, and 75 vol% Diamond/Cu composites were 50 µm and 400 µm
mixed. In order to prepare Diamond/Cu composites, Cu-Cr alloy was melted and poured into the
preform. The details of the pressure infiltration can be referred to elsewhere [6,7].
Ni-Au was plated on the surface of the Diamond/Cu composites by electroplating. The coating
consisted of a 5 µm Ni layer and 3 µm Au layer to ensure no copper or diamond was exposed.
The mechanically polished Diamond/Cu composites were sensitized and activated in a sensitizing
activation solution. Nickel plating was carried out using electroplating equipment at 82 ◦ C and pH 4.5
for 20 min, and Au was deposited using Di-propanedinitrile gold-based solution at 52 ◦ C and pH 5 for
15 min. The specific process is reported in reference [20].

2.2. Salt Spray Corrosion Test

Neutral salt spray experiments were carried out in a sealed test chamber (Scch-21, Singleton,
Cleveland, OH, USA). The polished samples were cleaned with anhydrous ethanol and deionized
water in turn, and then placed in the same horizontal position to ensure the same amount of corrosive
medium. The sample was exposed to a neutral salt spray environment in accordance with the national
standard GB/T 2423.17-93 [21]. The specific experimental conditions were: pH 6.5–7.2; salt spray
was from a neutral 5 wt% NaCl solution and was provided in a continuous manner at 35 ◦ C; the
deposition rate was 1.5–1.6 mL/h; the samples’ corrosion times were 16, 24, 48, 96, 168 h, respectively.
The sedimentation rate was determined using the specific method in accordance with GB/T 2423.17-93.
Materials 2020, 13, 1847 3 of 11

At any position in the salt spray box, a funnel with an area of 80 cm2 could collect 1.5 to 1.6 mL solution
per hour. From this, a deposition rate of 1.5 to 1.6 mL/h was determined. The sample collection surface
was placed horizontally in the salt spray box. Each sample was at the same distance from the salt
spray generator.

2.3. Corrosion Weight Loss Test

The mass loss of Diamond/Cu composites sample was tested using an electronic balance
(ME204T/02, 0.1 mg, METTLER TOLEDO, Zurich, Switzerland). The original weight of the samples
was weighed before the neutral salt spray test. After the neutral salt spray test, the sample was
immersed in a de-rusting solution (the de-rusting solution used was composed of 37% AR hydrochloric
acid and distilled water in a ratio of 1:1) for ultrasonic cleaning for 1 min to remove corrosion products,
followed by cleaning with deionized water and alcohol. Weighing the sample after de-rusting and
drying, the final corrosion weight loss was calculated by the following formula:

m1 − m2
v= (1)
S·t

v is the Diamond/Cu composite sample corrosion rate (g/(cm2 ·h));

t is the corrosion time (h);
m1 is the original weight (g);
m2 is the weight (g) of the sample after the neutral salt spray test and de-rusting;
S is the area (cm2 ) of the Cu matrix exposed to the corrosive medium.

In this paper, Image-Pro Plus 6.0 was used to calculate the exposed copper matrix area of the
Diamond/Cu composites. The copper matrix areas of 60 vol% and 75 vol% Diamond/Cu composites
accounted for 53.0% and 24.2% of the sample surface, respectively.

2.4. Characterization
The corrosion morphology of the composite was observed using a scanning electron microscope
JSM-7610FPlus of Hitachi, Tokyo, Japan. The extended depth of the corrosion interval of Diamond/Cu
composites was obtained using a micro-area scanning electrochemical workstation (Ametek, Berwin,
PA, USA, Versascan). The scanning setting was X-Y area, the scanning range was 6 × 6 mm, each step
was 100 µm, and the probe size used for the surface scanning was 100 µm. The thermal diffusivities of
the Diamond/Cu composites at room temperature were measured by a LFA447 thermal conductivity
tester of NETZSCH Company (Selb, Germany), and the sample size had a diameter of 12.6 mm and a
thickness of 2.5 mm. The bending strength of the samples was tested using an electronic universal
material testing machine (AG-250KNIS, Shimadzu, Kyoto, Japan) with a displacement rate of 0.5
mm/min. The composition and structure of the corrosion products of the samples were characterized
by X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS).

3. Results and Discussion

3.1. Surface Morphology and Microstructure

Figure 1 shows the surface macro-morphology of Diamond/Cu composites after different times in
a neutral salt spray environment. Visual inspection of the samples after the 168-h test revealed that the
surface of the Diamond/Cu composites was completely corroded. The light green substance on the
sample surface is the corrosion product. With the extension of corrosion time, the corrosion products
on the surface of the Diamond/Cu composites increased continuously. The corrosion phenomenon
mainly occurred on the copper matrix, and the diamond and interface products did not corrode [22].
When the neutral salt spray test lasted for more than 96 h, the corrosion products covered the entire
surface of the Diamond/Cu composites. The corrosion products of 75 vol% Diamond/Cu composites
Materials 2020, 13, 1847 4 of 11

Materials 2019, 12, x FOR PEER REVIEW 4 of 11

Materials 2019, 12, x FOR PEER REVIEW 4 of 11
were significantly less than those of 60 vol%. This is related to the exposed copper matrix area on the
copper matrix area on the sample surface. The copper matrix area of 60 vol% and 75 vol%
sample surface. The copper matrix area of 60 vol% and 75 vol% Diamond/Cu composites accounted
copper matrix
Diamond/Cu area on the
composites sample surface.
accounted for 53.0%Theand
copper matrix
24.2% of thearea of 60 surface
sample vol% and 75 vol% by
calculated
for 53.0% and 24.2% of the sample surface calculated by Image-Pro Plus. Since only copper corrodes,
Diamond/Cu
Image-Pro Plus.composites
Since onlyaccounted for 53.0%the
copper corrodes, and 24.2% of
amount of corrosion
the sampleproducts
surface depends
calculatedonbythe
the amount of
Image-Pro corrosion
Plus. Since products
only depends
copper on the
corrodes, the magnitude
amount of of the exposed
corrosion copper
products matrix on
depends area.
the
magnitude of the exposed copper matrix area.
magnitude of the exposed copper matrix area.

Figure Photographs
Figure1.1.Photographs of
Photographs of the
of the Diamond/Cu
the Diamond/Cu compositescorroded
composites
Diamond/Cu composites corrodedsurfaces
corroded surfaces
surfaces after
after
after salt
salt
salt spray
spray
spray testing
testing
testing for
for for
differenttimes.
different times.

Figure
Figure222shows
Figure shows
showsthe the micro-morphology
themicro-morphology
micro-morphology of ofthe
of theDiamond/Cu
the Diamond/Cucomposites
Diamond/Cu composites
composites including
including
including andand
and excluding
excluding
excluding
corrosion
corrosionproducts
corrosion products
products after thethe
after
after 48-h
the test.test.
48-h
48-h Corrosion
Corrosion
Corrosionproducts are distributed
products
products aredistributed
are on the
distributed substrate
ononthethe in a granular
substrate
substrate in in
a a
form, as
granular shown
form, in Figure
as shown
granular form, as shown in Figure 2a,d.
in The
Figureblack contrast
2a,d. The black contrast is diamond and the gray contrast as
The is
blackdiamond
contrast and
is the gray
diamond contrast
and the is copper
gray matrix,
contrast is is
shown
copper in Figure
coppermatrix,
matrix,as 2b,e.
asshown Diamond
shown in particles
in Figure and
Figure 2b,e. Diamond interface
Diamond product
particles
particles and Crinterface
and C2 (confirmed
3interface product
productbyCrprevious
3C23C
Cr research)
(confirmed
2 (confirmed
were
by substantially
byprevious
previous research) free were
research) of corrosion
were [6]. However,
substantially
substantially free
free of discontinuous
of corrosion
corrosion[6]. corrosiondiscontinuous
[6].However,
However, sites and cracks
discontinuous appeared
corrosion
corrosion
sites and cracks appeared on the copper matrix, indicating serious corrosion
sites and cracks appeared on the copper matrix, indicating serious corrosion of the copperinterface.
on the copper matrix, indicating serious corrosion of the copper matrix, of the
especially copper
at the matrix,
matrix,
especially
This
especially atthe
theinterface.
can beatattributed interface. This
to theThis can be
differences
can attributed
be in the physical
attributed to
to the
and
the differences
chemicalin
differences inthe
thephysical
properties of the
physical and chemical
materials
and and
chemical
properties
the of
heterogeneity the ofmaterials
the Cu-Cr and the
solid heterogeneity
solution of
composition the Cu-Cr
at the solid solution
interface
properties of the materials and the heterogeneity of the Cu-Cr solid solution composition at the [8,16].composition at the
interface[8,16].
interface [8,16].

Figure2.2.SEM
Figure SEMimage
image ofof 60
60 vol% Diamond/Cu
Diamond/Cucomposites
compositescontaining
containing (a)(a)
andand removing
removing (b)(b) corrosion
corrosion
products;
products; (c):
(c):partial
partialenlargement
enlargement ofof(b);
(b);SEM
SEMimage
image of
of75
75vol%
vol% Diamond/Cu
Diamond/Cu composites containing (d)
Figure 2. SEM image of 60 vol% Diamond/Cu composites containing (a) andcomposites
removing containing
(b) corrosion
and
(d) removing
and removing (e) corrosion products;
(e) corrosion (f) partial
products; enlargement
(f) partial enlargementof of
(e).(e).
products; (c): partial enlargement of (b); SEM image of 75 vol% Diamond/Cu composites containing
(d) and removing (e) corrosion products; (f) partial enlargement of (e).
Optical images and SEM images of the gold-plated Diamond/Cu composites’ corroded surfaces
after salt spray corrosion for 168 h are shown in Figure 3. The gold-plated layer on the surface of the
Optical images and SEM images of the gold-plated Diamond/Cu composites’ corroded surfaces
after salt spray corrosion for 168 h are shown in Figure 3. The gold-plated layer on the surface of the
Materials 2020, 13, 1847 5 of 11

Optical images and SEM images of the gold-plated Diamond/Cu composites’ corroded surfaces
afterMaterials
salt spray corrosion
2019, 12, x FOR PEERfor 168 h are shown in Figure 3. The gold-plated layer on the surface
REVIEW 5 of 11 of the
Diamond/Cu composites
Materials 2019, wasREVIEW
12, x FOR PEER uniform and compact. No corrosion products were observed on11the
5 of
Diamond/Cu
sample surface. Thiscomposites wasthat
indicates uniform
gold and compact.
plating No
on the corrosion
surface canproducts were improve
significantly observed the
on the
corrosion
sample surface.
Diamond/Cu
resistance of Diamond/CuThis indicates
composites wasthat gold
uniform
composites’ plating
and
surfaces. on the
compact. Nosurface canproducts
corrosion significantly
were improve
observed the
on the
corrosion
sampleresistance
surface. of Diamond/Cu
This indicates composites’ surfaces.
that gold plating on the surface can significantly improve the
corrosion resistance of Diamond/Cu composites’ surfaces.

Figure 3. Optical images and SEM images of the gold-plated Diamond/Cu composites corroded
Figure 3. Optical images and SEM images of the gold-plated Diamond/Cu composites corroded
surfaces after
surfaces salt
after spray
salt spraycorrosion
corrosion for 168hh((a)
for 168 ((a)6060vol%;
vol%;(b)(b)
75 75 vol%).
vol%).
Figure 3. Optical images and SEM images of the gold-plated Diamond/Cu composites corroded
surfaces after salt spray corrosion for 168 h ((a) 60 vol%; (b) 75 vol%).
3.2. 3.2.
Corrosion Depth and Corrosion Rate
Corrosion Depth and Corrosion Rate
3.2. Corrosion
Considering
Considering Depth
that
that and Corrosion
diamond
diamond does
does Rate
not participate
not in corrosion,
participate in corrosion, the area of Cuofexposed
the area was normalized.
Cu exposed was
Thenormalized.
corrosion weight
Considering loss
The corrosion curves
that weightof loss
diamond Diamond/Cu
curves
does not of composites
Diamond/Cu
participate with different
composites
in corrosion, thewith diamond
area of Cu content
different diamond
exposed after
was a 5
wt%content
NaCl neutral
after a 5salt
normalized. wt%
The spray
NaCltest
corrosion are shown
neutral
weight salt in Figure
lossspray test
curves 4. The
of are shown
Diamond/Cu corrosion rate4. of
in composites
Figure Diamond/Cu
The
withcorrosion composites
differentrate of
diamond
withDiamond/Cu
different composites
content diamond
after a 5 wt% with
NaCldifferent
contents decreased diamond
neutral salt spray contents
rapidly at first,
test decreased
and then
are shown rapidly 4. at
stabilized
in Figure Thefirst,
after and
48 hthen
corrosion corrosion.
rate of
stabilized after
Diamond/Cu 48 h corrosion.
composites This
with indicates
different that in
diamond the initial
contents stage of corrosion,
decreased rapidly
This indicates that in the initial stage of corrosion, the generation of corrosion products inhibited the generation
at first, and ofthenthe
corrosion
corrosion products
stabilized
occurrence. inhibited
after 48 At the
h corrosion.corrosion
the initialThis occurrence.
indicates
stage At
that in the
of corrosion, the
the initial
initial stage of
stage ofrate
corrosion corrosion,
corrosion, the corrosion
the generation
of 60 vol% Diamond/Cuof
ratecorrosion
of 60 vol% Diamond/Cu
products composites
inhibited the was higher
corrosion than that
occurrence. At of initial
the 75 vol% Diamond/Cu
stage of corrosion,composites.
the corrosion
composites was higher than that of 75 vol% Diamond/Cu composites. As the corrosion progressed,
As the
rate corrosion
of 60 vol%progressed,
Diamond/Cu thecomposites
corrosion rates of the than
was higher two that
wereofsimilar.
75 vol%This indicates composites.
Diamond/Cu that the
the corrosion rates of the two were similar. This indicates that the copper matrix corrosion rates of
copper
As the corrosion progressed, the corrosion rates of the two were similar. This indicatesclose.
matrix corrosion rates of Diamond/Cu composites with different diamond contents are that the
Diamond/Cu composites with different diamond contents are close.
copper matrix corrosion rates of Diamond/Cu composites with different diamond contents are close.

Figure 4. Corrosion rate of Diamond/Cu composites with different diamond content in neutral salt
spray
Figure 4.test.
Figure 4. Corrosion
Corrosion rate
rate of of Diamond/Cu
Diamond/Cu composites
composites withdifferent
with different diamond
diamond content
contentininneutral saltsalt
neutral
spray test.
spray test.
Materials 2020, 13, 1847 6 of 11

Materials 2019, 12, x FOR PEER REVIEW 6 of 11

As the probe swept across the sample, the sample surface and corrosion cracks produced different
As the probe swept across the sample, the sample surface and corrosion cracks produced
signals. Corrosion depth can be calculated by the height difference of multiple scans in Figure 5b,d.
different signals. Corrosion depth can be calculated by the height difference of multiple scans in
The average of multiple
Figure 5b,d. scanning
The average depths was
of multiple the average
scanning depths corrosion depth. The
was the average extended
corrosion depth
depth. The of the
corrosion pits of Diamond/Cu composites obtained by the micro-scanning electrochemical
extended depth of the corrosion pits of Diamond/Cu composites obtained by the micro-scanning workstation
is shown in Figure 5.
electrochemical The meaniscorrosion
workstation shown indepths of The
Figure 5. composites with different
mean corrosion depths diamond contents
of composites with were
different
similar. diamondcorrosion
The average contents depths
were similar. The average
of 60 vol% corrosion
and 75 vol% depths of composites
Diamond/Cu 60 vol% andwere 75 vol%
30 and 35
µm, Diamond/Cu
respectively. composites were from
As can be seen 30 andFigure
35 μm,5c,d,
respectively.
there areAs can be
more seenpeaks
sharp from Figure
in the 5c,d,
OSP there are
(Non-Contact
more sharp peaks in the OSP (Non-Contact Surface Profiling) scan results. It shows
Surface Profiling) scan results. It shows that 75 vol% Diamond/Cu composites have more and deeper that 75 vol%
Diamond/Cu composites have more and deeper corrosion sites, indicating that 75 vol%
corrosion sites, indicating that 75 vol% Diamond/Cu composites have more severe local corrosion.
Diamond/Cu composites have more severe local corrosion.

Figure 5. Corrosion
Figure 5. Corrosiondepth
depthofofDiamond/Cu compositesafter
Diamond/Cu composites after salt
salt spray
spray corrosion
corrosion for h:
for 168 168(a)h:3D(a) 3D
topography of 60ofvol%
topography Diamond/Cu
60 vol% composites;
Diamond/Cu (b) corrosion
composites; interval
(b) corrosion depth of
interval 60 vol%
depth of 60Diamond/Cu
vol%
Diamond/Cu
composites; (c) 3Dcomposites;
topography (c) of
3D75topography of 75 vol%
vol% Diamond/Cu Diamond/Cu
composites; (d) composites; (d) corrosion
corrosion interval depth of 75
vol%interval depth ofcomposites.
Diamond/Cu 75 vol% Diamond/Cu composites.

3.3. Performance
3.3. Performance after
after Tests
Tests
TheThe bending
bending strengthofofDiamond/Cu
strength Diamond/Cu composites
compositesafter thethe
after neutral saltsalt
neutral spray test is
spray shown
test in
is shown in
Figure 6. With the prolongation of corrosion exposure time, the strength of the
Figure 6. With the prolongation of corrosion exposure time, the strength of the Diamond/Cu compositesDiamond/Cu
composites fluctuated around the initial value (red dashed/dotted line) before the test, and the
fluctuated around the initial value (red dashed/dotted line) before the test, and the fluctuation range
fluctuation range was less than 5%. There was no significant increase or decrease in the bending
was less than 5%. There was no significant increase or decrease in the bending strength of Diamond/Cu
strength of Diamond/Cu composites as the corrosion progressed. Therefore, it can be considered that
composites
the changeas in
thebending
corrosion progressed.
strength Therefore,
was mainly caused byitthe
can be considered
difference that the
of the sample change
itself. in bending
The neutral
strength was mainly caused by the difference of the sample itself. The neutral salt
salt spray test had little effect on the bending strength of Diamond/Cu composites. This was spray test had little
effectconsistent
on the bending
with the strength of Diamond/Cu
performance composites.
change of Diamond-Cu This was
composites in aconsistent with the performance
humid environment [17,23].
change of Diamond-Cu composites in a humid environment [17,23].
Materials 2020, 13, 1847 7 of 11
Materials 2019, 12, x FOR PEER REVIEW 7 of 11
Materials 2019, 12, x FOR PEER REVIEW 7 of 11

Figure Bending
6. 6.
Figure strength
Bending of of
strength Diamond/Cu composites
Diamond/Cu during
composites neutral
during saltsalt
neutral spray test.
spray test.
Figure 6. Bending strength of Diamond/Cu composites during neutral salt spray test.
AsAsshown
shown in in
Figure
Figure7, 7,
after thethe
after 168-h
168-hneutral
neutral salt spray
salt spraytest, thethe
test, thermal
thermal conductivity
conductivity of of
6060vol%
vol%
and 75 As shown
vol% in Figure
Diamond/Cu 7, after the
composites 168-h neutral
without surface salt spray decreases
treatment test, the thermal
by 142.9conductivity
and 92.8 W/mK,of 60and
vol%
and 75 vol% Diamond/Cu composites without surface treatment decreases by 142.9 and 92.8 W/mK,
and
theand 75 rates
decay vol%reach
Diamond/Cu and composites
22%reach 12%, without surface treatment decreases by 142.9 and 92.8 W/mK,
the decay rates 22% respectively. The
and 12%, respectively.thermal conductivity
The thermalofconductivity
gold-plated Diamond/Cu
of gold-plated
and
compositesthe decay
decreased rates reach 22%
by 9.8 decreased and
and 9.2 W/mK, 12%, respectively.
respectively. The
The effect thermal conductivity
of neutralThesalt effect of gold-plated
spray corrosion
Diamond/Cu composites by 9.8 and 9.2 W/mK, respectively. of neutralon salt
Diamond/Cu
thespray
thermal composites
conductivity decreased
of thermal
Diamond/Cu by 9.8 and
composites 9.2 W/mK, respectively. The effect of neutral
such assalt
corrosion on the conductivity of was mainly caused
Diamond/Cu by surface
composites wasroughness,
mainly caused by
spray corrosion
corrosion on on the thermal conductivity ofcorrosion.
Diamond/Cu composites wasinterface
mainly caused by
surface cracks
roughness, the surface
such of the
as corrosion sample
cracksafter
on the surface of It did
the not affect
sample the
after corrosion.bonding
It did not
surface
state roughness, such as corrosion
of Diamond/Cu cracks
theon the surface of the showed
sample after corrosion. Itdown
did not
affect the interfacecomposites.
bonding state As of
a result,
Diamond/Cu thermal conductivity
composites. As a result, thea slight decrease,
thermal conductivity
affect
1.6% the
and 1.2%, interface bonding
respectively. state
After of
the goldDiamond/Cu composites. As a result, the thermal conductivity
showed a slight decrease, down 1.6%plating treatment,
and 1.2%, althoughAfter
respectively. the partial
the goldthermal conductivity
plating treatment,
showed
of although a
the Diamond/Cu slight decrease,
composites down 1.6% and 1.2%, respectively. After the gold plating treatment,
the partial thermal was lowered, of
conductivity thethe
surface state andcomposites
Diamond/Cu performance wasafter corrosion
lowered, theof the
surface
although
composite the
were partial
greatly thermal
improved. conductivity of the Diamond/Cu composites was lowered, the surface
state and performance after corrosion of the composite were greatly improved.
state and performance after corrosion of the composite were greatly improved.

Figure 7. Thermal conductivity of Diamond/Cu composites before and after 168-h neutral salt spray
Figure
Figure 7. Thermal
7. Thermal conductivity
conductivity of Diamond/Cu
of Diamond/Cu composites
composites beforebefore and168-h
and after after neutral
168-h neutral salttest.
salt spray spray
test.
test.

3.4. Corrosion Mechanism

3.4. Corrosion Mechanism
Due to the different physical and chemical properties of diamond and metal substrates, there
Due to the different physical and chemical properties of diamond and metal substrates, there
were many interfacial regions with different properties. In addition, the surface roughness of the
were many interfacial regions with different properties. In addition, the surface roughness of the
Materials 2020, 13, 1847 8 of 11

3.4. Corrosion Mechanism

Due to the different physical and chemical properties of diamond and metal substrates, there8 were
Materials 2019, 12, x FOR PEER REVIEW of 11
many interfacial regions with different properties. In addition, the surface roughness of the sample
after mechanical
sample polishing was
after mechanical Ra = 1–2
polishing µmRa
was measured
= 1–2 by μmroughness
measured meter: higher roughness
by roughness meter: resulted
higher
in smaller pits on the matrix surface. These places would provide preferential
roughness resulted in smaller pits on the matrix surface. These places would provide preferential attack in the salt spray
environment. Corrosion pits first appeared at the interface and the pits
attack in the salt spray environment. Corrosion pits first appeared at the interface and the pits of matrix, and finally some of
corrosion
matrix, andcrevices
finallyformed
some [24]. Therefore,
corrosion the formed
crevices corrosion mechanism
[24]. Therefore, of Diamond/Cu
the corrosioncomposites
mechanismwas of
micro-galvanic corrosion.
Diamond/Cu composites was micro-galvanic corrosion.
The EDS
The EDS analysis
analysis ofof the
the corrosion
corrosion products
products of of Diamond/Cu
Diamond/Cu composite samples after
composite samples after 168-h
168-h
corrosion showed
corrosion showed that that the
the corrosion
corrosion products
products contained
contained O, O, Cl,
Cl, and
and CuCu elements
elements (Figure
(Figure 8). The
8). The
corrosion product is confirmed to be Cu Cl(OH) by XRD as shown in
corrosion product is confirmed to be Cu22Cl(OH)33 by XRD as shown in Figure 9. The electrochemicalFigure 9. The electrochemical
reaction that
reaction that occurred
occurred was was analyzed
analyzed based based on on the
the corrosion
corrosion product.
product.
Anode reaction: Cu →
Anode reaction: Cu → Cu + 2e Cu 2+
2+ + 2e−
−

Cathodic reaction:
Cathodic 1/2O22 ++HH2O
reaction: 1/2O 2 O++ 2e2e
−
− →→2OH 2OH −
−

Salt spray
Salt spray isis aa dispersion
dispersion system
system consisting
consisting of of many
many tinytiny droplets
droplets of of sodium
sodium chloride. When the
chloride. When the
Diamond/Cu composite was in this environment, it was easy to form
Diamond/Cu composite was in this environment, it was easy to form a thin water film containing a a thin water film containing
a largeamount
large amountofofsodiumsodiumchloride
chlorideon onthe
thesurface
surfaceofofthe thesample,
sample, and and the
the sample
sample was was subjected
subjected to to
electrochemical etching.
electrochemical etching. Matrix
Matrix copper
copper entered
entered the the solution
solution in in the
the form
form of of hydrated
hydrated ions,ions, leaving
leaving
electrons in the metal, which flowed from the anode to the cathode. O
electrons in the metal, which flowed from the anode to the cathode. O2 reached the cathode surface reached the cathode surface
− 2+
by diffusion or convection
convection to to absorb
absorb the theremaining
remainingelectrons
electronsininthe themetal
metaltotoform
formOH OH. . Cu
− Cu formed
2+

corrosion product Cu2Cl(OH) Cl(OH)33 with

withCl Cl−−and
andOH OH− −ininthe thesolution
solution[16,25].
[16,25].
The electrode reaction and corrosion process can be inferred by determining the composition of
corrosion products. The The salt
salt spray
spray formed
formed an an electrolyte
electrolyte film on the sample surface, which provided
the necessary conditions for electrochemical corrosion. corrosion. O2 in in solution
solution played
played an important
important role in the
corrosion process. At At the initial stage of corrosion, the sample sample surface
surface waswas completely
completely exposed.
exposed. O O2
easily reached
reachedthe themetal surface
metal surface through
through diffusion, resulting
diffusion, in a faster
resulting in a corrosion rate. With
faster corrosion theWith
rate. increase
the
of corrosion
increase products,products,
of corrosion the diffusionthe of O2 , Cl− of
diffusion was O2limited,
, Cl− wasand the corrosion
limited, and therate decreased.
corrosion Eventually
rate decreased.
diffusion reached
Eventually a dynamic
diffusion reachedequilibrium,
a dynamic and the corrosion
equilibrium, and rate remained essentially
the corrosion rate remained unchanged.
essentially
unchanged.

Figure
Figure 8.
8. EDS
EDS analysis
analysis of
of the
the corrosion
corrosion products.
products.
Materials 2020, 13, 1847 9 of 11
Materials 2019, 12, x FOR PEER REVIEW 9 of 11

Figure XRD
9. 9.
Figure pattern
XRD ofof
pattern Diamond/Cu composites
Diamond/Cu corrosion
composites products.
corrosion products.
4. Conclusions
4. Conclusions
Diamond/Cu composites were prepared by pressure infiltration. This paper investigated the
Diamond/Cu composites were prepared by pressure infiltration. This paper investigated the
thermal conductivity, mechanical properties, and corrosion behavior of the Diamond/Cu composites
thermal conductivity, mechanical properties, and corrosion behavior of the Diamond/Cu composites
in a neutral salt spray environment. Based on the experimental results, the following conclusions
in a neutral salt spray environment. Based on the experimental results, the following conclusions
were drawn:
were drawn:
1. A salt spray environment will seriously damage the surface morphology of Diamond/Cu
1. A salt spray environment will seriously damage the surface morphology of Diamond/Cu
composites. The corrosion product can cover the entire surface after 48-h tests. Localized corrosion is
composites. The corrosion product can cover the entire surface after 48-h tests. Localized corrosion is
induced along the diamond-matrix interface. As the corrosion time prolongs, microcracks are created
induced along the diamond-matrix interface. As the corrosion time prolongs, microcracks are
at the interface, and then the corrosion begins and propagates throughout the surface of the composite.
created at the interface, and then the corrosion begins and propagates throughout the surface of the
2. After the 168-h neutral salt spray test, the thermal conductivity of 60 vol% and 75 vol%
composite.
Diamond/Cu composites without surface treatment decreased by 142.9 and 92.8 W/mK, respectively.
2. After the 168-h neutral salt spray test, the thermal conductivity of 60 vol% and 75 vol%
The thermal conductivity of gold-plated Diamond/Cu composites decreased by 9.8 and 9.2 W/mK,
Diamond/Cu composites without surface treatment decreased by 142.9 and 92.8 W/mK, respectively.
respectively. There was no significant increase or decrease in the bending strength of Diamond/Cu
The thermal conductivity of gold-plated Diamond/Cu composites decreased by 9.8 and 9.2 W/mK,
composites as the corrosion progressed. The strength of Diamond/Cu composites fluctuated less than
respectively. There was no significant increase or decrease in the bending strength of Diamond/Cu
5% compared to that of the initial value.
composites as the corrosion progressed. The strength of Diamond/Cu composites fluctuated less
3. Surface metallization is an effective measure to improve the corrosion resistance of Diamond/Cu
than 5% compared to that of the initial value.
composites in a neutral salt spray environment, and can effectively improve the surface state and
3. Surface metallization is an effective measure to improve the corrosion resistance of
thermal conductivity of the composite.
Diamond/Cu composites in a neutral salt spray environment, and can effectively improve the
4. The corrosion mechanism of Diamond/Cu composites is micro-galvanic corrosion. The corrosion
surface state and thermal conductivity of the composite.
product formed is Cu2 Cl(OH)3 .
4. The corrosion mechanism of Diamond/Cu composites is micro-galvanic corrosion. The
corrosion
Author product formed
Contributions: is Cu2Cl(OH)
Conceptualization, 3. and Z.X.; methodology, Z.X.; investigation, X.Z.; data curation,
H.G.
S.H.; writing—original draft preparation, Z.X.; writing—review and editing, H.G.; funding acquisition, H.G. All
authors
Authorhave read and agreed
Contributions: to the published
Conceptualization, version
H.G. of themethodology,
and Z.X.; manuscript. Z.X.; investigation, X.Z.; data curation,
S.H.; writing—original
Funding: This research wasdraft preparation,
funded Z.X.;of
by the Ministry writing—review and editing,
Science and Technology H.G.;
of China, funding
grant acquisition,
number H.G.
2016YFB0301402.
Acknowledgments: The authors
Funding: This research acknowledge
was funded the support
by the Ministry in electrochemical
of Science experiments
and Technology of China,given
grant by Ms.
number
Wenmei Zhang.
2016YFB0301402.
Conflicts of Interest: The authors declare no conflict of interest. The authors would like to declare that the
Acknowledgments: The authors acknowledge the support in electrochemical experiments given by Ms.
work described was original research that has not been published previously, and not under consideration for
Wenmei Zhang.
publication elsewhere, in whole or in part.
Conflicts of Interest: The authors declare no conflict of interest. The authors would like to declare that the work
described was original research that has not been published previously, and not under consideration for
publication elsewhere, in whole or in part.

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Materials 2020, 13, 1847 10 of 11

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