MULTI-SCALE AND MULTI-PHASE NANOCOMPOSITE CERAMIC TOOLS AND CUTTING PERFORMANCE

An advanced ceramic cutting tool material Al2O3/TiC/TiN (LTN) is developed by incorporation and dispersion of micro-scale TiC particle and nano-scale TiN particle in alumina matrix. With the optimal dispersing and fabricating technology, this multi-scale and multi-phase nanocomposite ceramic tool material can get both higher flexural strength and fracture toughness than that of Al2O3/TiC (LT) ceramic tool material without nano-scale TiN particle, especially the fracture toughness can reach to 7.8 MPa·m0.5. The nano-scale TiN can lead to the grain fining effect and promote the sintering process to get a higher density. The coexisting transgranular and intergranular fracture mode induced by micro-scale TiC and nano-scale TiN, and the homogeneous and densified microstructure can result in a remarkable strengthening and toughening effect. The cutting performance and wear mechanisms of the advanced multi-scale and multi-phase nanocomposite ceramic cutting tool are researched.     

Cutting performance of Si<Subscript>3</Subscript>N<Subscript>4</Subscript> based SiC ceramic cutting tools

Composites of Si₃N₄-SiC containing up to 30 wt% of dispersed SiC particles were fabricated via hot-pressing with an oxynitride glass. To determine the effect of sintering time and SiC content on the mechanical properties and the cutting performance, the composites with fixed 8 hr-sintering time and 20 wt% SiC content were fabricated and tested. Fracture toughness of the composites increased with increasing sintering time, while the hardness increased as the SiC content increased up to 20 wt%. The hardness of the composites was relatively independent of the grain size and the sintered density. For machining heat-treated AISI4140, the insert with 20 wt% SiC sintered for 8 hr showed the longest tool life while the insert with 20 wt% SiC sintered for 12 hr showed the longest tool life for machining gray cast iron. An effort was made to relate the mechanical properties, such as hardness, fracture toughness and wear resistance coefficient with the tool life. However, no apparent relationship was found between them. It may be stated that tool life is affected by not only the mechanical properties but also other properties such as surface roughness, density, grian size and the number of the inherent defects in the inserts.     

On the use of fractal geometry methods for the wear process characterization

Irregular fluctuations of friction coefficient around the mean value were analysed by means of fractal geometry methods. Particular data were recorded during the steady-state stage of dry sliding tests carried out in Si₃N₄ ball - on - Si₃N₄/SiC nanocomposite disc configuration. It was proven that the set of points {sliding distance (time), friction coefficient}, considered as a geometric object in a two-dimensional space, has the property of a fractal curve. The fractal dimension of this curve increased with increasing wear rate gained in a particular wear test. This could indicate a possible correlation between the wear rate and the fractal dimension of friction coefficient as a function of sliding distance (time).