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Abstrak :
High velocity oxy-fuel (HVOF) sprayed tungsten carbide ? cobalt (WC-Co) coatings exhibit attributes that allow them to be a candidate material for high temperature applications; such as temperature insulators for rocket nozzles. This application note investigates the effect of surface preparation, in this case the grit blasting process, on the characteristics of the so-formed coating. The WC-Co coatings exhibited high hardness and low porosity. The composition of WC-Co coating varied in different regions, but on average was close to the composition of the initial feedstock, implying that there was no preferential loss of the material during the spray process. Microanalysis indicated diffusion of tungsten to the interface between the coating and the substrate and partly explains the high bonding strength of the coating. These physical characteristics suggest that the HVOF sprayed WC-Co is an appropriate coating technology for rocket nozzles.

Abstrak :
Pada makalah ini dibahas karakteristik lapisan WC-Co hasil proses thermal spray dengan teknik HVOF (High Velocity Oxy-Fuel) yang akan diaplikasikan pada pada nosel roket RX-10Q guna menggantikan pelapis grafit masif yang berat. Untuk itu, dilakukan studi pengaruh kekasaran permukaan terhadap kekuatan lekat dan struktur mikro lapisan. Hasil yang diperoleh menunjukkan bahwa variasi tekanan proses penyemprotan (grit blasting) menyebabkan variasi kekasaran permukaan, dimana semakin besar tekanan yang diberikan, semakin kasar permukaan yang diperoleh. Namun, kekuatan kohesi lapisan dan kekerasan lapisan WC-Co tidak dipengaruhi oleh kekasaran permukaan tersebut. Hasil penelitianjuga menunjukkan bahwa tanpa proses persiapan permukaan, lapisan WC-Co thermal spray tidak dapat melekat dengan baik pada logam dasarnya. Analisa struktur mikro lapisan WC-Co menggunakan mikroskop optik dan Scanning Electron Microscope (SEM) menunjukkan bahwa lapisan WC-Co relatifpadat dengan komposisi bervariasi pada daerah berbeda. Terdapat indikasi terjadinya difusi W pada daerah antar-fasa yang berkontribusi pada tingginya kekuatan lekat lapisan. ......This research investigated the characteristics of HVOF (High Velocity Oxy-Fuel) thermal spray WC-Co coating to be applied on the nozzle of RX-100 rocket. In particular, it studied the effects of surface preparation on the mechanical properties and microstructures of the coating. The results showed that the grit blasting pressure directly influences the surface roughness of the substrate, in -which the greater the pressure, the higher the surface roughness. However, the cohesive strength and hardness of the coating are not affected by the surface roughness. This research also showed that without grit blasting, the WC-Co coating can not be applied on the substrate. Microstructural analysis of the WC-Co coating showed that this coating is relatively dense with variety in chemical content on different area. An indication of the diffusion of Win the interfacial area was found, which is thought to contribute to the high strength of the coating.