Substrate molybdenum

The borides are extremely hard (9.8—29 GPa (1000—3000 kgf/mm ) Knoop) and, in the case of molybdenum, >39 GPa (4000 kfg/mm ) (see Hardness). However, oxidation resistance is usually poor unless a subsequent coating is formed, such as silicidi ing or chromizing, which imparts oxidation resistance. SiUcides are generally very oxidation resistant, but not as hard as borides. SiUcide coatings formed on molybdenum (51 pm in 3 h) at 675°C have superior oxidation resistance. At these low temperatures, the molybdenum substrate does not embrittle and the coatings are quite flexible. 

Similar tests were then carried out with a variety of metal sulphides on six different metal substrates, and the results are shown in Table 6.2. Again they showed that the lives were consistently better with molybdenum substrates, for all the sulphides tested. This strongly suggested that some chemical interaction was taking place, possibly between free sulphur or sulphur compounds and the molybdenum in the substrate. Where the original sulphide under test was itself molybdenum disulphide, this reaction would represent a re-supply mechanism. Where some other sulphide was under test, the reaction would again enable molybdenum disulphide to be formed which would supplement the life of the original sulphide. 

Rhenium layers with (0001) preferred orientation were fabricated on molybdenum substrates by chemical vapor deposition (CVD). Molybdenum has light weight, high thermal conductivity and good workability compared to other refractory metals. 

It was found that tungsten is an effective diffusion prevention layer, which improves the thermal stability of the rhenium layer by reducing the amount of chemical diffusion between the rhenium layer and the molybdenum substrate. The tungsten layer was deposited by reduction of WF using hydrogen, at a substrate temperature of 900 1100 K, and a reaction pressure of 1.3 kPa. 

Diffusion was promoted by heating the samples in a vacuum of about 6.5X10 Pa at 2000 2300 K in order to form a composition gradient between the molybdenum substrate and the rhenium layer. The composition gradient was investigated using an electron probe microanalyzer (EPMA). 

Based on these results, a (0001) oriented rhenium thermionic emission layer ( 1(X)// m ), and a tungsten diffusion prevention layer (5(X) /im) were formed on a molybdenum substrate, and emitters with a composition gradient at each interface were produced. 

A gradient structure emitter, in which the molybdenum substrate is covered by a ((XX)1) rhenium layer with a three-dimensional surface and an intermediate tungsten layer was fabricated, and the work function and thermionic power generation characteristics were evaluated. The following results were obtained. 

A CdSexTei x powder of any desired composition is prepared according to Section A. Molybdenum substrates of any suitable size for painting of 1 cm2 with CdSexTe x are cleaned as described for Ti in Section B. The preoxidizing step required for Ti (and Cr) is unnecessary here, although also not detrimental. 

B-N-Ti ternary ceramics are either deposited by thermal CVD from BCI3, TiC, NH3, and H2 on graphite substrates [20, 21], or by sputtering boron and titanium in an Ar/N2 atmosphere to coat a molybdenum substrate [22]. 

Figure la. The initial stage of molytxlenum growth onto (Oil) plane of molybdenum substrate. x200... 

Figure 6. Etch pits of dislocations with increasing thickness of molybdenum deposit on the (001) plane of molybdenum substrate (a,b) c- the twin defect formation. x200 a - substrate, b - molybdenum deposit (thickness - 0.05mm) c - molybdenum deposit (thickness - 0.1mm).

It was shown that epitaxial growth of molybdenum and tungsten onto bent monocrystalline molybdenum substrates with different orientations and radius of curvature occured during electrodeposition from oxide and halide melts. The morphology of bent growth surface corresponded to the morphology of platelike growth surface. 

The coatings in question were obtained by electrochemical deposition in (Li-Na-K)F-K2NbF and (K-Na)Cl-K2NbF7 melts with a soluble anode. Their thicknesses were 5-200 pm. The current cathode density U was varied over the interval 50-1000 A/m2. The deposition was conducted at 970-1070 K onto flat pieces (30x50 mm) and cylinders (0 6x60 mm) of copper and molybdenum substrates, which were separated from the coatings to be studied.The equipment, the reactants and preparation of the latter were basically the same as in earlier papers [5,6]. 

As follows from XRD analysis, MoNi and MoNi4 alloys form on the surface of molybdenum plates during both currentless transfer and electrolysis. The alloy formation leads to a loosening of the molybdenum substrate surface, which increases the specific surface area of the samples during carbonization. 

Curves of Figure 19 compare the data published for (a) boron nitride [37,40] (b) aluminium (c) diamond-[37-39] (d) aluminium nitride [37-42] (e) crystalline silica. It can be seen that, at 45 vol.%, the maximum thermal conductivity achieved with diamond powder is 1.5 W m K, while crystalline boron nitride at 35 vol.% affords 2.0Wm K. The thermal conductivity of silver-filled adhesives was studied by using silicon test chips attached to copper and molybdenum substrates [43]. The authors outline the importance of the shape factor A, related to the aspect ratio of the particles, to achieve the highest level of thermal conductivity. Another study reports the variation of the effective thermal resistance, between a test chip and the chip carrier, in relation to the volume fraction of silver and the thickness of the bond layer [44]. The ultimate value of bulk thermal conductivity is 2 W m at 25 vol.% silver. However, the effective thermal conductivity, calculated from the thermal resistance measurements, is only one-fifth of the bulk value when the silicon chip is bonded to a copper substrate. 

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