Boride layers

Fig. 4.9. Formation of a molybdenum boride layer between Mo and B-containing phase (a) and between Mo and MoB (b)289 Annealing temperature 1450°C. Time (a), 9000 s (2.5 h) (b), 7200 s (2 h). Photographs kindly provided by Prof. W. Lengauer. Reprinted with permission from Elsevier Science.

Among the hexavalent elements, molybdenum and tungsten seem to be the most technologically important metals because of their use as deposits on steels in the form of protecting metal or metal boride layers. Since the main features of molybdenum and tungsten electrolytes are similar, the following text will deal with the properties of the former. 

Boron carbide (B4C). In the contact zone between the two components, tungsten boride layers can be detected after heat treatment between 1100 and 1600 °C. Depending on the reaction conditions, a-WB or W2B is formed. Additions to NaF as activator. 

V. Passage of gaseous mixtures consisting of a metal halide, BBra and Hg over incandescent carrier metals on the average yields only boride layers inhomogeneous, and solid products. 

High Performance Coating of Steel with Tantalum Boride Layers... 

The coating thickness was determined by means of metallographical examination of cross sectioned parts. The thickness (d) of the boride layer correlated with the exposure time has a dependence ... 

Fig. 5 Ta coated(left) steel-bolt (polished) Fig 6 boride layer, Ta-coating, interface, steel...

The liquid acid was examined by means AAS spectroscopy for boron. If no residuels were detected, the qualification of no attack (-) was determined. Slight attack is a change in colour, but no real etching. The partial dissolution of tantalum in comparison to the boride layer after exposure to boiling HF for three hours is visible in the remaining boride rim around the tantalum (Fig. 11). Attack after 6 min. could be seen by macroscopic inspection. 

By TEM- analysis of refractory boride layers the formation kinetics have been formulated [13]. The first stage of layer formation occurs by monoboride precipitation at preferred orientation by forming needle shaped orthorhombic crystals. Superlattices enable the formation of more boron enriched complexes where the boron concentration of the most stable diboride is exceeded during the diffusion process [14]. As boriding acts as a thermochemical process controlled by diffusion [1, 9,11], this study has developed a new procedure between solid and gaseous state treatment [9]. Further developments show that activation by BF3 at low pressure can be used for plasma processes [15]. 

Under vacuum conditions the thermal stability of boride layers is very high exceeding temperatures of 2500 K, and remaining extremely hard [6]. Oxydative environments reduce this stability dramatically to temperature limits of 900K where the boride dissolves. A positive aspect of this enhanced reactivity in air is the possibility of recycling substrate material by oxy dative tempering at 1000 K. 

Vetters H., et al.. TEM analysis of microcrystalline boride layers, Microstructural Science (ASM internat. Publ. Metals Park Ohio 1988), 16,499-507... 

Mechanical properties of tantalum boride layers 24 Mechanism of molybdenum electrodeposition 186... 

Fig. 4. Micro structure of borided layer on the EN CIS steel consisting of FeB (dark) and FezB (light) phases.

Fig. 5. Relationship between the boride layer thickness and time for iron and steel boronized with B4C-Na2B4CVNa3AlF6 based paste at 1000 °C.

Our investigations on peck boriding show that it is pessible to change the traditional type and amount of activator, and provide a successful diffusion process with high quality of obtained boride layers. The compositions of pewder mixtures were from 55 % B4C 1 to 3 % NaBF4 or NasAlFe and diluter AI2Q3. The thermochemical treatments are carried out with... 

Fig. 6. Microstructure of boride layers on C60 steel obtained for 2 (a) and 6 (b) hours p>ack boriding at 950 °C in powder mixture containing 3 % NasAlFe (xl50).

KBF4 is an additive in boronizing baths, which are used for the surface hardening of tools by [57] providing a boride layer on carbon steel. 

Boe] Boehmann, G., Klaus, E., Fritsche, G., Wagnre, W., Phase Structure of Boride Layers on Iron Substrates with Eleetroplated Cobalt Layers , Crysi Res. Techn., 22(7), 961-967 (1987) (ExperimentaL Morphology, 14)... 

Bad] Samples from Cr-Fe alloys containing Fe-0.5 mass% Cr and Fe-4 mass% Cr were subjected to boriding at 1000°C. Boride layers of Fc2B and FeB were found and their textine was studied. 

Dyb] SEM/EPMA, XRD, abrasive wear resistance test, microhardness. Samples from a melted alloy Fe-10 mass% Cr and mixture of B and KBF4 were annealed at 850, 900 and 950°C Boride layers of Fc2B and FeB were found to form between a Fe-10 mass% Cr and B at 850 to 950°C. The kinetic of their growing was studied. 

Bra] Penetration depth measiuements Boride layer growth kinetics... 

---