Microstructures and mechanical properties of thin films of aluminum oxide

Abstract

Alumina has been widely used as a wear- resistant coating for tribological applications because of its good abrasion resistance, chemical inertness, dimensional stability, resistance to thermal shock, and mechanical strength at high temperatures. In cutting tools, for example, coatings of alumina with layer thicknesses in the range 5 to 15 {mu}m are applied in conjunction with TiC, TiN, Ti(C,N) or HfN to cemented carbides to improve the service lives of the tools. Alumina is known to exist in a number of metastable polymorphic forms. These include the thermodynamically stable alpha-Al{sub 2}O{sub 3} phase ({alpha}, hexagonal, or corundum), gamma ({gamma}, cubic spinel), delta ({delta}, tetragonal), theta ({theta}, monoclinic), eta ({eta}, cubic spinel), kappa ({kappa}, orthorhombic), chi ({chi} cubic), beta ({beta}, hexagonal), and iota ({iota}, ). For wear applications, only the {alpha}-Al{sub 2}O{sub 3} or {kappa}- Al{sub 2}O{sub 3} phases are desirable because of their relatively high hardness. Alumina coatings can be produced by a number of techniques including physical vapor deposition (PVD), chemical vapor deposition (CVD), anodization, and molecular beam epitaxy. In the past, wear-resistant alumina coatings have been made primarily by CVD because this technique is relatively efficient and capable of producing films with uniform coverage. The phases which formmore » in alumina thin films depend upon the processing techniques and conditions, and the phase transformation sequence can take place by various routes. By PVD techniques, for example, a commonly reported transformation sequence is amorphous {yields} {gamma} {yields} ({theta} + {delta}) {yields} {alpha}, in which the transformation products at each stage may be a mixture of several phases.« less