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Aluminum Coating on Ceramic Using the High Pressure Cold Spray Process

ASB Industries provides Research & Development for many unique customer projects with many efforts going into Cold Spray technology where a wide range of evolving materials are being applied to common to unique substrates. As new ideas evolve for actual applications ASB finds the need to do internal studies that allow us to offer possible processing that customers may be able to use these ideas to evolve their own applications. Our internal capabilities include access to material suppliers, equipment modifications that may become commercialized and our membership to technical societies and with journal access. One such example is the application of metallic coatings onto ceramic substrates.

The following information is directly documented by Dr. Reza Daroonparvar who is ASB’s Director of Research and Development:

Introduction

Isolating material substrates that can be joined with metals with high electrical conductivity are common applications in power electronic applications. Isolated gas bipolar transistors, metal-oxide transistors and analogous conductive structures on electrically isolating materials are common applications. The metallization of ceramics can be a challenge due to considerable differences in the properties of coating materials and ceramic substrates. Metallization of technical ceramics such as Al2O3, AlN, SiC, Si3N4 is mostly accomplished using complex methods such as physical vapor deposition, chemical vapor deposition, direct copper bonding. Thermal spray processes have been substantiated to be an option and possibly a simpler methods for spraying metals on the ceramics. However, formation of oxides, residual tensile stresses and phase transformation in thermal sprayed coatings may constrain the applications in the field of electrical and electronic engineering.

Cold Spray Coating Processing

High Pressure Cold Spray uses kinetic energy to produce the coating layer rather than a combination of thermal and kinetic energies such as HVOF spray. In the cold spray process, fine powder particles are propelled toward the substrate surface with supersonic velocity. Upon impact, the particles undergo adiabatic heating and plastically deform at very high shear rates, which leads to flattening and bonding them to the underlying surface. Cold spray operating temperatures are very low compared with those of thermal and plasma spray methods. The low operating temperature of cold spray together with the use of inert gas prevents oxidation of the particle surfaces, phase transformation, grain growth, and formation of undesirable phases in the coating during spraying. This can substantially help for the production of pure and dense coatings of oxidation-sensitive metals.

The Technical team at ASB Industries has developed cold spraying pure metals on to ceramic substrates such as AlN. No obvious micro-cracks or micro-pores can be found at the Al coating/Alumina substrate interface as seen below in the SEM images. The following pictures show experimental results:

Alumina Substrate with Aluminum Cold Spray