Hot Isostatic Pressing Modifications of Pore Size Distribution in Plasma Sprayed Coatings∗

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Abstract
Plasma sprayed coatings contain relatively large amount of pores. This is primarily due to the nature of deposition by the liquid droplets upon impact. This paper reports the modifications made in the pore size distribution of plasma sprayed yttria stabilized zirconia (YSZ) and Ti-6Al-4V/hydroxyapatite (HA) composite coatings following hot isostatic pressing (HIP). The pore size distribution was measured by a mercury intrusion porosimeter (MIP). The results indicated that the YSZ coatings which were HIPed for 1 hour and 3 hours in the temperature range 1000° to 1200°C and ∼185 MPa showed a small decrease in the average porosity (∼2.5%) for the 1 hour samples. However, the hardness increased ∼39%, and there was a corresponding increase in the coating density. This was due to reduction of the average pore size in the HIPed coatings. Thus, in the YSZ coatings, the pores responded to HIP by a general breakdown of large pores to smaller ones and effectively forming many ‘new’ interparticle contacts. Whilst the overall porosity was reduced marginally, the increase in physical property like hardness was significant because of the increase in interparticle and inter-lamellae contacts following HIP treatment. In the Ti-6A1-4V/HA composite coatings, the reduction of pores is most significant amongst the small pores. The porosity of the as sprayed 20 wt% HA composite was ∼19%. This value was reduced to 17% for the sample HIPed at 1,000%C for 1 hour. Although the reduction was relatively minor, the interesting aspect was the drastic reduction of small pores less than 0.3 μm. The average pore diameter was observed to increase from 0.1676 μm in the as sprayed coating to 0.787 μm in the sample HIPed at 1,000°C, as a result of the elimination of the micro-pores. Physical properties such as microhardness, Young's modulus and density increased substantially. This is believed to be aided mainly by the plastic deformation of the ductile Ti-6A1-4V phase during HIP. Thus modification of the pore size distribution or even average pore size can elicit substantial improvement in the properties in two different material coating, albeit the difference in the manner the modification occur.