难熔金属及其合金低温流床硅化物防护层

在700—1050℃温度的流床中,W、Nb和Ta的硅化物层生长速率单调地随温度上升而增加。流床法获得的硅化物层与其它方法得到的相类似。 Mo及Mo—Ti、Mo—Ti—C和Mo—Ti—Zr—C合金的硅化行为则不同,在较低的温度区域内,它们的硅化物层生长速率出现反常高的峰值,该峰的出现伴随着形成一个在现有的二元Mo—Si系相图中没有报导过的新相组成的硅化产物。X-射线分析确定,该相具有典型的六角C_(40)型结构,点阵常数为:a=4.580,c=6.477,c/a=1.41。它是通常四角的MoSi_2相的一个不稳定的低温变体。硅化后经高温真空处理时,六角的MoSi_2迅速地和不可逆地转变为四角的MoSi_2。 在出现反常高增重峰的温度下,Mo一0.5Ti合金硅化物层生长遵守直线规律,其硅化层主要为六角的MoSi_2。而纯Mo仅在硅化初期服从直线规律,而后过渡为抛物线规律,其硅化产物通常为六角的MoSi_2,随后转变为四角的MoSi_2。两种结构的硅化物层在高温时的氧化行为相同。

The Silicide Coating of Refractory Metals and Their Alloys in a Fluidized-Bed at Relatively Low Temperatures

The rate of growth of silicide coatings on W, Nb and Ta in a fluidizedbed at 700-1050℃ increases with temperature as a simple function. The silicide coatings obtained in the fluidized-bed are similar to those prepared by other methods. However, Mo behaves differently as do its alloys such as Mo-Ti, Mo-Ti-C and Mo-Ti-Zr, etc. At relatively low temperatures, an abnormal peak of silicide growth rate appears, accompanied by a new phase unreported in the Mo-Si phase diagram. It has a hexagonal crystalline structure of C_(40) type, with lattice parameters a=4.580A,c=6.477A, and c/a=1.41. It is a metastable low temperature modification of the normal tetragonal MoSi_2.When treated in vacuum at high temperatures after siliciding, the hexagonal MoSi_2 converts rapidly and irreversibly into the tetragonal form. At abnormal peak temperature, the growth of the silicide coating on Mo-Ti obeys a linear law, and the silicide coating consists mainly of hexagonal MoSi_2. On the contrary, when pure Mo is silicided, the rate of growth obeys a linear law only at the very beginning, but then shifts approximately to a parabolic law. The silicide coating changes correspondingly from hexagonal to tetragonal MoSi_2. Both forms, however, have the same oxidation behavior at high temperatures.