Effects of Diamond Content on the Thermal Conductivity of Copper Matrix Composite Materials Prepared by Cold Spraying

With the increasing demand for rapid heat transfer in electronic packaging materials, quickly transferring the large amount of heat generated is an urgent problem that needs to be solved in current electronic information manufacturing. Diamond is added to copper based composite materials to further improve the thermal conductivity of electronic packaging materials. In this paper, copper matrix composite materials were selected as the research object, and the effects of diamond content on the microstructure and thermal conductivity were systematically investigated. As the content of copper plated diamond in the mixed powder increases, the proportion of copper plated diamond in copper based composite materials gradually increases, but the deposited copper plated diamond exhibits fragmentation phenomenon. Increasing the content of copper plated diamond leads to a rapid decrease in thermal conductivity, and subsequent process optimization and heat treatment are needed to unleash the thermal conductivity of diamond.


Introduction
With the increasing demand for rapid heat transfer in electronic packaging materials, further improving the thermal conductivity of electronic packaging materials and rapidly transferring a large amount of heat is an urgent problem to be solved in current electronic information manufacturing [1,2].It is an effective method to adjust the thermal expansion coefficient and thermal conductivity of composite materials by adding reinforcing phases with lower thermal expansion coefficients with higher thermal conductivity to copper [3,4].And in order to achieve a breakthrough in the thermal conductivity of metal matrix composites, it is necessary to find reinforcing phases with higher thermal conductivity.[5].Diamond materials have excellent thermophysical properties, such as ultra-high thermal conductivity (1200-2000W/(m• K)) and low thermal expansion coefficient, has received widespread attention from scholars both domestically and internationally [6][7][8][9].
However, for diamond/copper composites, the central problem limiting the fulfilment of their high thermal conductivity is the poor wettability of copper and diamond, even in the molten state copper is difficult to wet diamond, which leads to cracks and voids between diamond and copper matrix in diamond/copper composites, and poor interfacial bonding [10][11][12].Therefore, improving diamond/copper interfacial bonding is a critical issue in the preparation of diamond/copper composites.In previous studies, small diamond additions have not shown an improvement in the thermal conductivity of the composites [13].Therefore, in this paper, copper matrix composites with different diamond additions were prepared by cold spraying technique and the effects of diamond additions on microstructure and thermal conductivity were investigated.
This article takes high thermal conductivity copper based composite materials as the research object, and systematically studies the effect of diamond content on microstructure and thermal conductivity, providing reference for the development of diamond reinforced high thermal conductivity copper based composite materials.

Experimental
The substrate material is copper (over 99.9% pure, 20×5×0.5cm).The spraying powder is a mixture of pure copper powder and copper plated diamond (20-50μm.wt.% C=83.3%), and the mass ratios of copper-plated diamonds in the powder mixes used for the different samples were 20%, 40%, 60% and 80%, respectively.Copper matrix composite materials are prepared using cold spraying equipment (PCS-1000).The carrier gas in the experiment is pure helium (20Mpa.Purity>99.9%),the carrier gas temperature is 500 °C , the working distance is 20 mm, the powder gas flow rate was 300 SLM, and the carrier gas pressure is 5 MPa.
The substrate materials were cut off by wire cutting to obtain copper based composite material after spraying.Then thermal conductivity test samples and metallographic samples were cut from the composite material.The thermal conductivity and microstructure studies of the samples are the same as in previous articles [13].

Powder morphology
Figure 1 demonstrates the SEM images of diamond and pure copper powders.Diamond powders are mainly in block form, with a size of about 20-50 μm, and a thin layer of copper can be seen on the surface.The copper powders are mainly spherical in shape with a size of 5-60 μm.

microstructure
Figure 2 shows the microstructure of copper matrix composites with different diamond additions.The white substrate is deformed copper, and black diamonds can be seen evenly distributed in the substrate.The diamond particles and the copper matrix are previously well bonded with no visible cracks.Comparing figure 4(a) and 4(d), the number of diamond particles in the Cu-80% C sample is about 3-4 times that of the Cu-20% C sample, indicating that increasing the proportion of copper plated diamond in the mixed powder significantly increases the diamond content in the copper based composite material.Figure 3 shows that SEM images of diamonds in copper matrix composite.It is evident that the grain boundaries of the copper matrix around the diamond are elliptical.During cold spraying process, copper powder particles undergo severe plastic deformation during high-speed deposition, forming layers of elliptical copper layers.The increase in copper-plated diamond content did not have a significant effect on the microstructure of the copper substrate.When the amount of copper-plated diamond in the powder is low, the diamond particles are relatively intact.As the content of copper plated diamond increases, the diamond particles undergo fragmentation, especially for Cu-80% C samples, a large number of small-sized diamond blocks appear.This may be due to the collision between copper plated diamonds during cold spray preparation.When the content of copper plated diamond in the powder is low, more copper plated diamond is deposited on the copper substrate.Due to the relatively soft nature of copper, copper plated diamond particles are more easily embedded in it.When the content of copper plated diamond in the powder increases, there are more collisions between copper plated diamonds, leading to some of the copper plated diamonds breaking during deformation.And it may lead to a decrease in the deposition rate of copper plated diamond.

Thermal Conductivity
Figure 4 demonstrates the thermal conductivity of the copper matrix composite with different amounts of copper-plated diamond additions.As the content of copper plated diamond increases from 20% to 80%, the thermal conductivity of the composite decrease rapidly, decreasing by about 60%.This indicates that increasing the content of copper plated diamond in the powder will harm the thermal conductivity of the composite, that is, the high thermal conductivity of diamond is not utilised, which may be related to the fragmentation of diamond during deposition.As mentioned earlier, diamond particles undergo fragmentation, especially when the copper plated diamond is relatively high.The broken diamond particles increase the phase interface, and their thermal conductivity may also decrease.Therefore, subsequent optimization of process parameters and heat treatment will be carried out to reduce the degree of fragmentation of diamond particles and reduce phase interfaces.

Conclusions
This paper investigates the effect of diamond content on the thermal conductivity of copper matrix composites.The following conclusions can be drawn: (1) The composite consists of a deformed elliptical copper matrix layer and diamond particles.which are more closely bonded to the copper and diamond.
(2) As the content of copper plated diamond in the mixed powder increases, the proportion of copper plated diamond in copper based composite materials gradually increases, but the deposited copper plated diamond exhibits fragmentation phenomenon.
(3) For copper matrix composites prepared by cold spraying, the increase in diamond content leads to a rapid decrease in thermal conductivity, which may be related to the fragmentation of diamond and the increase in phase interface.

Figure 1 .
Figure 1.SEM images of diamond and copper powders.

Figure 2 .
Figure 2. The microstructures of copper matrix composite with different diamond additions.

Figure 3 .
Figure 3. SEM images of diamonds in copper matrix composite.

Figure 4 .
Figure 4.The thermal conductivity of the copper matrix composite with different amounts of copperplated diamond addition.