TZM alloy has good high temperature mechanical property, but at a high temperature oxidation resistance property is poor. Under high temperature, it unable to form a protective oxide layer, so it must be used at high temperatures in non-oxidizing atmosphere. When the temperature is below 400 ℃, TZM alloy oxidation rate is very slow, because alloy surface formed less volatile MoO2. When the temperature is between 400 ~ 750 ℃, the oxidation rate becomes faster, the alloy surface to generate volatile MoO3. When the temperature is higher than 750 ℃, due to MoO3 volatile oxide weight decrease sharply.
Improving alloy’s oxidation resistance usually has two methods: alloying and coating method. Alloying method can not provide sufficient high-temperature oxidation resistance. Therefore the more commonly used method is coating method.
There are many coating production process. The pack cementation method has many advantages including low cost, easy control, strong combination, so it has been widely used. TZM alloy coating by pack cementation method, generally choose Al powder as coating materials, NH4Cl as catalyst, Al2O3 as filler. The optimum parameters for the coating growth is tm (Al2O3): m (Al): m (NH4Cl) = 7: 2: 1,1000 ℃, 12h.
Observing XDR found that coating consist of Al5Mo, Al2Mo and Al2 (Mo4) 3 phase. Near the substrate the coating thickness is about 30um and average Al content is lower. Outer coating thickness is 20um and average Al content is higher. TZM alloy under coating protective within 10h operation, the oxidation rate is very fast. But within 10 ~ 50h, the oxidation rate becomes slowly, because the Al2O3-layer has a better protective effect and can prevent oxidation further development. It is shows coating method can well improve the oxidation resistance of the TZM alloy.