Irradiation-induced shrinkage of highly crystalline SiC fibers

Studies of ion beam mixing involving insulator substrates is reviewed with an emphasis on thermodynamical and thermochemical considerations. Such studies generally have employed the bi-layer configuration of metal/insulator combinations. There is little evidence for long-range material transport. In the case of sapphire (single crystal Al2O3) substrates, only recoil mixing has been detected for both metal and oxide films irradiated at room temperature. “Demixing” of recoil-implanted cations is indicated at temperatures of 825–900°C. Agglomeration of non-wetting metal films suggests that surface diffusion occurs under the influence of the ion beam. Ballistic mixing has been reported for metal films on SiC, Si3N4 and SiO2. Ion beam-induced reactions are observed at the interfaces for systems in which the enthalpy of reaction is favorable. There is evidence that chemical effects in the cascade mixing regime determine the phases present in the mixed zone.

Ion beam mixing in insulator substrates

Surface modification of ceramics has proved to be of benefit in a number of applications. Excimer laser light provides a rapid and efficient means for surface alloying and modification of ceramic materials. The laser pulse can melt the surface of the ceramic as well as a deposited metal layer. This results in diffusional mixing in the liquid state, which is followed by rapid resolidification. The high temperatures involved can overcome kinetic barriers to mixing found in ion beam experiments. Tribological measurements on surface treated materials show reduced friction and increased surface toughness. Increased fracture toughness in laser surface alloyed ceramics has also been observed. Excimer laser processes therefore may enable the use of ceramic materials in applications for which they are not now suitable.

Excimer laser surface processing of ceramics: Process and properties

Excimer laser mixing of nickel overlayers on silicon carbide

Pulsed excimer laser radiation has been successfully employed in the improvement (> 50%) of fracture strength of metal-coated ceramics. Thin metallic layers (∼500 A) of nickel were deposited on silicon nitride and silicon carbide substrates and further irradiated with pulsed excimer (xenon chloride, krypton fluoride) laser pulses. The laser energy density was varied from 0.4 to 2.0 J cm −2 to optimize the formation of mixed interfacial layers. The formation of interfacial layers was studied by transmission electron microscopy and Rutherford backscattering spectrometry techniques. Detailed heat flow calculations using implicit finite difference methods were performed to simulate the effects of intense laser irradiation on metal-coated ceramic structures. Three different mechanisms were found to play an important role in the improvement in the fracture strength of these ceramics. Theoretical calculations showed that the displacement of the crack tip away from the free surface by laser surface modification can lead to a 100% improvement in the fracture strength of the ceramic.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400003538

Laser surface modification of metal-coated ceramics

We have investigated ion‐beam and laser mixing of Ni overlayers on silicon carbide substrates using cross‐section and analytical techniques of electron microscopy, and ion scattering and channeling techniques. The thickness of the overlayers was varied from 200 to 500 A on both crystalline and polycrystalline substrates of silicon carbide. The thickness of the ion‐beam mixed region was found to increase with increasing dose of the implanted ions. The laser mixing was obtained only within a critical window of the pulse energy density (1.15±1.25 J cm−2) of a ruby laser (wavelength 0.693 μm, and pulse duration 28×10−9 s). The structure of the mixed region was found to be amorphous, with occasional presence of crystalline nickel silicide (Ni2Si) islands. The laser‐mixed SiC specimens showed a considerable improvement (>35%) in their fracture strength.

P. F. Becher

Papers

Ion‐beam and laser mixing of nickel overlayers on silicon carbide