Doctor of Philosophy
Department of Materials Engineering
Huang, Shao Wei, Tribological performance and microstructure of physical vapour deposition coatings, laser clad WC/Ni layers and duplex coatings, Doctor of Philosophy thesis, Department of Materials Engineering, University of Wollongong, 2000. http://ro.uow.edu.au/theses/1536
This work is aimed at developing a surface engineering solution for applications requiring resistance to both abrasive and adhesive wear. The central plank of this work was to deposit and characterise duplex coatings by applying thin, hard films of titanium carbo-nitrides on a pre-clad metal matrix composite (MMC) coating of tungsten carbide in nickel matrix formed by a laser cladding process.
In the first tranche of the project pulsed laser cladding of WC/Ni MMC with a Nd:YAG laser and optical fibres was investigated with the aim of producing a clad layer with minimum porosity, crack and dilution but with a sound metallurgical bond to the tool steel and stainless steel substrates. Thorough microstructural (XRD, optical and electron microscopy), mechanical (hardness) and abrasive wear characterization of the clad layers enabled optimisation of laser beam parameters, type and composition of powders and substrate preparation. It was established that WC/Ni MMC prepared from injected crushed powder can substantially improve the abrasive wear resistance of the substrate, almost by a factor of 10.
In the second tranche of the project a ftltered arc deposition system was utilised to deposit hard coatings of titanium nitride, carbo-nitrides and carbide. Controlled variation of the partial pressure of reactive gases (nitrogen and methane) by mass flow controllers monitored by a mass spectrometer allowed deposition of adherent, macroparticle-free fllms with any desired carbon to nitrogen ratio, on austenitic stainless steel substrates.
The mechanical properties (microhardness and adhesion) of the coatings were determined using an Ultra-Micro Indentation System (UMIS-2000) and adhesion scratch tester, respectively. The adhesive wear resistance of PVD coatings was evaluated by a pin-ondisc wear tester. It was established that higher concentration of carbon increases the hardness, reduces the adhesion but increases the adhesive wear resistance.
In the third tranche of the project, duplex coatings combining a PVD film deposited on a pre-clad WC/Ni layer were produced on stainless steel substrates. The duplex coatings exhibited similar hardness and adhesive wear results to those of the PVD coatings, but with a signifIcant increase in load bearing by a factor of 4-5, confirming the pivotal role of clad layer to support the applied load.