DEVELOPMENT OF A CEMENTATION COATING PROCESS FOR HIGH-TEMPERATURE PROTECTION OF MOLYBDENUM

A pack cementation coating process was successfully developed to protect molybdenum alloy entry vehicle components against oxidation at temperatures near 3000 deg F. Designated PFR-6, the 2 to 3 mil diffusion coating is a silicide prepared from a siliconizing pack containing small amounts of niobium powder. It was evaluated statistically in an Analysis of Variance study to demonstrate an average lifetime of 1.36 hr in an oxyacetylene torch at a temperature exceeding 3000 deg F. The 95% confidence limit is 0.71 to 2.01 hr at the same temperature. Three major process concepts were studied: single-cycle codeposition, multicycle codeposition, and multicycle alternate layer coatings. Using these processes 41 types of coatings were made and screened initially, from which 5 were selected for further evaluation. An intensive study of processing and testing variables employing a formal statistical approach resulted in the choice of the single- cycle PFR-6 coating for process optimization. Pack composition, process time, and process temperature were optimized. Recommendations are made for scale-up development of the coating process. (auth)