Microstructure and Wear of CVD Coatings used in Metal Cutting Applications
This thesis is concerned with the microstructural characterisation of wear resistant Al2O3 and Ti(C,N)coatings produced by chemical vapour deposition on cemented carbide cutting tools. The investigation focuses on the effect of deposition conditions on the microstructure and the microstructural evolution of the coatings during heat-treatments and metal cutting. New characterisation techniques and their application to these materials are evaluated. Transmission electron microscopy was used extensively throughout the work and imaging, electron diffraction, energy dispersive X-ray spectroscopy, electron energy-loss spectroscopy and energy-filtering were utilised.
The growth rate of .kappa.-Al2O3 can be increased with proper deposition pressures in combination with H2S catalysis,without resulting in drastic changes in microstructure. A limiting factor is homogeneous gas phase nucleation at higher pressures,which leads to inhomogeneous coating thicknesses. High H2S concentrations are unsuitable for growth of .kappa.-Al2O3 at low temperatures,as this stimulates the nucleation and growth of .gamma.-Al2O3.
Tools coated with moderate temperature chemical vapour deposited Ti(C,N)are successful in metal cutting due to low reduction in tool toughness after coating and improved edge chipping resistance. The twinned columnar microstructures of the coatings result from slow kinetics and low desorption rates of reaction products from selected crystal surfaces. The introduction of CO in the process leads to grain refinement, which yields a high surface smoothness and low friction coefficient.
Heat-treatment of .kappa.-Al2O3 coatings results in more fine-grained .alfa.-Al2O3 coatings than is generally produced by chemical vapour deposition. These transformed coatings feature increased porosity, which is due to the volume decrease at the phase transformation. When .kappa.-Al2O3 coated tools are used in turning, a transformation to .alfa.-Al2O3 may occur. This transformed .alfa.-Al2O3 has a smaller grain size than heat-treated coatings and shows no porosity, which is due to the high temperature and pressure at the transformation. Rake face wear of .kappa.-Al2O3 coated tools occurs mainly as plastic deformation of transformed .alfa.-Al2O3 , while flank face wear occurs as abrasive wear of .kappa.-Al2O3.
The increased machinability of Ca-treated Al-killed steels is mainly an effect of softer and less abrasive inclusions, while adhering inclusion layers play only a minor role in reducing tool wear. The layers consist of high melting point phases of the inclusion population in the workpiece material. No evidence of reactions between inclusion layers and Al2O3 coatings could be found.
In addition to the use for quantitative microanalysis of light elements, the power of electron energy-loss spectroscopy in determinations of structural and electronic properties has been shown in this work.