Yun Kon Joo

Friction Behavior of HVOF Thermal Spray Coating of Micron Size WC-Co Powder

High velocity oxy-fuel (HVOF) spray coating of WC base cermet has been studied as a promising candidate in some hard coating area for a replacement of the long traditional electrolytic hard chrome (EHC) plating, which has been raising public health and environmental problems. Micron size WC-Co (mWC-Co) has been coated on Inconel 718 substrate by HVOF thermal spray coating process. Surface properties of coating layer and friction behavior have been investigated for durability improvement coating on the sliding machine component surface.

Effects of pressure on metal atom transport and plasma properties during arc ion plating of TiAlN

Arc ion plating is used to deposit titanium and aluminum nitrides as wear and corrosion resistant layers on the surface of steel tools. In this study, the deposition efficiency for the evaporated metal atoms in arc ion plating was obtained by a view factor solution for the collisionless transport and a diffusion solution for the continuum transport. In the diffusion solution, the deposition efficiency decreases rapidly with pressure. A global nitrogen plasma model with a set of simplified plasma chemistry was used to investigate the plasma properties at pressures ranging from several mTorr up to 50 mTorr. The results indicate that the electron temperature, the ratio of electric field to gas density, and the nitrogen atom density all decrease with increasing pressure, while the plasma density increases. Based on the combined considerations of deposition efficiency and effective ion bombardment of the films on the substrate, an operating pressure range from 20 to 30 mTorr is recommended for highly efficient (Ti, Al)N coating during film deposition.

Wear and Corrosion Resistances of Inconel718, HVOF Coating of WC-Metal Powder and Laser Heat-Treated Coating

For the Improvement of wear and corrosion resistances of Inconel718 (In718) surface, high velocity oxygen fuel (HVOF) thermal spray coating of micron-sized WC-Cr-C-Ni powder was coated onto Inconel718 surface and laser heat-treatment of the coating was carried out. Porous coating of porosity 2.2±0.4% was prepared by HVOF coating, and it was improved by laser heat-treatment, reducing the porosity to 0.35±0.08%. Micro-hardness of laser heat-treated coating increased more than four times compared to the surface of In718. Friction coefficient decreased by HVOF coating and laser heat-treatment. Wear resistance improved, decreasing the wear depth by the coating and laser heating. The interface between coating and In718 was compacted, and elements diffused from both coating and inconel718 substrate to interface, forming metal rich buffer zone (interface) and enhancing the adhesion of coating. Corrosion resistance improved by coating in sea water 3.5% NaCl solution and in 1M HCl acid, but it worsened in 1M NaOH base. For the improvement of wear and corrosion resistances of Inconel718, HVOF WC-metal power coating and laser heat-treatment are recommended.

A Study on the Properties, Friction, Wear and Adhesion of HVOF Thermal Spray Coating of Micron Size Co-Alloy Powder

High velocity oxygen fuel (HVOF) thermal spray coating of micron size (μ) T800 powder has been studied for the durability improvement of sliding machine components. The optimal coating process (OCP), surface properties, friction, wear behavior and adhesion of HVOF T800 coating have been investigated. The temperature dependence of friction coefficient and wear behavior have been studied at 25°C and at an elevated temperature 538°C (1,000°F) for the study of the temperature effects on FC and wear behaviors of the coating and for the application on high speed air bearing spindle which operates with no lubricants. The OCP was determined from the best surface properties of the 16 OCP searching coatings designed by the Taguchi experimental program of four spray parameters with three levels: a hydrogen flow rate (FR) of 38-42 FMR (1 FMR=12scfh=9.44×10-5 m/s), oxygen FR of 65-70 FMR and feed rate of 30 g/min, and a spray distance of 5 inch. Hardness, roughness and porosity observed from the 16 coatings were 560-640 Hv (5488-6272 MPa), 2.2-3.0 μm and 0.01-0.04% respectively. Friction coefficient (FC) decreased from 5.5-7.0 to 4.8-6.0 with increasing the sliding surface temperature from 25°C to 538°C because of the higher lubrication effect of Co oxide debris at the higher temperature. Wear trace of the coating and counter sliding SUS 304 surface decreased to more than a half with increasing the sliding surface temperature from 25°C to 538°C. Tensile bond strength (TBS) and tensile fracture location (TFL) of Ti64 / T800 were 8,770 psi (60.5 MPa) and near the middle of the coating respectively. Bond coat NiCr did not influence on the TBS of the coating. The adhesion between Ti64 substrate and T800 coating (Ti64/T800) was stronger than the cohesion strength 8,770 psi (60.5 MPa) of T800 coating. These showed that Ti64/T800 coating was recommendable for durability improvement coating on high speed spindle of Ti64.