Tribological properties of carbonitride coatings formed on titanium by thermodiffusion carbonitriding

We have studied the tribological properties of VT14 titanium alloy with carbonitride coatings formed by the contact and noncontact method and binary (oxide and nitride) coatings. In the case of the contact method, specimens are saturated in a graphite backfill, and in the case of the noncontact method, specimens are located above it. We have investigated the wear resistance of a “titanium disk–bronze block” friction pair in AMH-10 hydraulic fluid under a load to 3 MPa on a friction path to 15 km. It has been established that coatings based on ternary interstitial compounds (titanium carbonitrides) provide a higher wear resistance than that of coatings based on binary interstitial compounds (titanium nitrides and oxides).     

Influence of fretting corrosion on the fatigue life of structural elements made of VT6 alloy

We present results of an experimental investigation of fatigue resistance characteristics of VT6 titanium alloy plates with washers made of D16AT aluminum alloy, VT3-1 titanium alloy, and 30KhGSA steel and pressed to them. It is established that, under operating loads, the fatigue life of titanium specimens decreases 400 – 750 times as a result of fretting corrosion.     

Properties and structural-phase state of the surface layers of titanium after combined nitriding

We study the simultaneous nitriding of titanium alloys by two methods: thermodiffusion saturation and ion implantation. Prior to nitrogen implantation, a thin oxynitride film and a thick nitride one were formed on the surface of VT6 titanium alloy of the (α + β)-class (Ti-6Al-4V). We show that nitrogen implantation changes the state of the surface of titanium and increases the surface microhardness of nitride and oxynitride coatings. An increase in the hardness of the surface without loss of its quality is possible in the case of nitrogen implantation into a thin oxynitride film. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 3, pp. 65–70, May–June, 2007.     

Structure and properties of electric-spark alloyed TiN,TiB<Subscript>2</Subscript>, and Mo coatings on VT22 titanium alloy and their transformation under light-thermal treatment

We have established that electric-spark alloyed TiN, TiB₂, and Mo coatings improve substantially the tribotechnical characteristics of VT22 titanium alloy. It has been shown that an increase in the slip velocity from 1.0 to 10 m/sec increases the wear intensity and friction coefficient by a factor of 1.5–2 and 1.3, respectively. A significant amount of oxides in the coatings and their additional alloying with titanium after treatment by concentrated solar radiation favor a 3–4 times increase in the layer thickness but lower its tribotechnical properties.     

Fatigue characteristics of VT22 titanium alloy with wear-resistant coatings

We study the influence of wear-resistant gas-thermal plasma and ion-plasma nitride coatings, diffusive electrochemical treatment, and vacuum nitriding on the fatigue resistance of specimens made of VT22 titanium alloy. The experimental results demonstrate that the wear-resistant hard coatings decrease the fatigue limit, whereas the procedure of sand-blasting increases the fatigue resistance of the alloy. Unlike diffusive coatings, plasma coatings suffer cracking and exfoliate from the surface of titanium alloy, which reveals their insufficient adhesion and high stiffness. The obtained results are quite urgent for the aircraft industry, where the VT22 alloy is extensively used. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 6, pp. 119–122, November–December, 2006.     

Crack resistance of the near-weld region of welded joints of titanium and aluminum alloys

We studied the crack resistance K_c (K_Ic) of the near-weld region of welded joints of VT3-1 and VT14 titanium alloys and AMg6NPP, 12401, and 1420 high-strength aluminum alloys under fatigue and static loading. By the methods of fracture mechanics and according to the results of testing cylindrical specimens with external circular cracks initiated in the weld metal, fusion zone, zone of thermal influence, and parent material, we established that the minimum crack resistance is observed in the fusion zone and the maximum crack resistance in the parent material. The weld metal and the zone of thermal influence are characterized by intermediate values of crack resistance both under fatigue and static loading. By using 1402 high-strength aluminum alloy as an example, we studied the influence of the texture, procedure of welding, modes of thermal hardening, and scaling factor on the character of brittle fracture of the metal in the near-weld region. To enhance the characteristics of crack resistance of the near-weld region of welded joints in its weakest zones (weld and fusion zone), we propose efficient methods for welding these alloys, e.g., electron-beam welding together with the efficient choice of the modes of thermal hardening instead of argon-arc welding. This enables one to improve the purity and homogeneity of the structure of the metal in the molten-metal bath in the process of welding by decreasing its porosity and the number of inclusions and cracks appearing in these zones. “L’vivs’ka Politeknika” State University, L’viv, Ukraine. Translated from Problemy Prochnosti, No. 5, pp. 89–99, September–October, 1998.     

Fatigue of VT20 titanium alloy with vacuum-plasma coatings at high temperatures

We study the influence of vacuum–plasma TiN, (TiAl)N, and (TiC)N coatings on the high-cycle fatigue of VT20 titanium alloy in the temperature range 350–640°C for a loading frequency of 10 kHz. It is shown that, in this temperature range, the fatigue limits of VT20 alloy with the indicated coatings 6 μm in thickness become 15–25% higher than for the material without coating. The possibility of replacement of steel blades with titanium blades with vacuum-plasma coatings is demonstrated. Translated from Problemy Prochnosti, No. 4, pp. 101–107, July–August, 2009.     

Physicochemical properties of electric-spark coatings based on VT6 titanium alloy

We establish regularities of the process of formation and physicomechanical and physicochemical properties of the surface of VT6 titanium alloy subjected to electric-spark alloying with electrode materials based on refractory compounds of titanium and zirconium. It is shown that, in the process of electricspark alloying, the maximum hardness and electrochemical resistance of the alloyed layer in natural corrosive environments are attained for materials based on carbide and titanium carbonitride and the highest resistance to high-temperature oxidation is attained for materials based on zirconium nitride and titanium carbonitride. Frantsevich Institute of Problems in Materials Science, Ukrainian Academy of Sciences, Kiev. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 5, pp. 55–59, September–October, 1996.     

Long-term assessment of the implant titanium material—artificial saliva interface

This paper studies the long-term (20,000 exposure hours) behavior of titanium and Ti–5Al–4V alloy—Carter–Brugirard saliva interface and the short-term (500 exposure hours) resistance of titanium and Ti–5Al–4V alloy—Tani&Zucchi saliva interface. Potentiodynamic polarization method was applied for the determination of the main electrochemical parameters. Linear polarization measurements for to obtain the corrosion rates were used. Monitoring of the open circuit potentials (E _oc) for long-term have permitted to calculate the potential gradients due to the pH, ΔE _oc(pH) and to the saliva composition ΔE _oc(c) changes which can appear “in vivo” conditions and can generate local corrosion. Atomic force microscopy (AFM) has analyzed the surface roughness. Ion release was studied by atomic absorption spectroscopy (AAS). In Carter–Brugirard saliva both titanium and Ti–5Al–4V alloy present very stable passive films, long-term stability, “very good” resistance, low values of the open circuit potential gradients, which cannot generate local corrosion. In Tani&Zucchi artificial saliva, pitting corrosion and noble pitting protection potentials (which cannot be reached in oral cavity) were registered; titanium ion release is very low; surface roughness increase in time and in the presence of the fluoride ions, denoting some increase in the anodic activity.     

Production of conversion oxide-ceramic coatings on zirconium and titanium alloys

We consider the possibilities of surface hardening of zirconium and titanium alloys with conversion oxide-ceramic coatings. These coatings have been produced by the method of plasma-electrolytic treatment in alkaline solutions. We have established that the plasma temperature in discharge spark channels reaches (6–9) · 10³ K. The thickness of the coatings is 100 to 120 and 30 to 40 μm, and their microhardness is ∼ 800 and ∼ 1000 MPa for zirconium and titanium alloys, respectively. The functional properties of the coatings depend on the synthesis conditions, including the electrolyte composition, the cathode and anode current densities, and also the treatment time. We have evaluated the thickness, microhardness, and wear resistance of the coatings under conditions of dry friction and cavitation as well as their fatigue strength and corrosion resistance. We have established that this treatment provides a high wear and corrosion resistance of the alloys under study with insignificant decrease in their fatigue strength. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, pp. 117–124, March–April, 2006.     

The effect of quenching medium on the wear behaviour of a Ti–6Al–4V alloy

The aerospace alloy, Ti–6Al–4V is a difficult material to machine, and, in general, shows poor wear resistance due to the soft, ductile properties of the alloy. In this study, the Ti–6Al–4V alloy has been heat treated to a temperature above and below the β-transus temperature and then quenched using a medium of oil, water or liquid nitrogen to change the surface wear behaviour of the alloy. The results showed that no significant change in microstructure and surface properties was achieved when the alloy was heated to 750 °C and then quenched in liquid nitrogen. However, when the alloy was heated to 1,000 °C (above the β-transus), the hardness of the titanium alloy significantly increased from 400 VHN to about 800 VHN, but the wear resistance of the alloy did not improve. In fact, the wear resistance of the alloy decreased as the surface hardness increased, and this change in wear behaviour was attributed to a change in the mechanism of wear from plastic deformation to brittle-fracture of the surface.     

Wear characteristics of spray formed Al-alloys and their composites

In the present investigation, different Al based alloys such as Al–Si–Pb, Al–Si, Al–Si–Fe and 2014Al + SiC composites have been produced by spray forming process. The microstructural features of monolithic alloys and composite materials have been examined and their wear characteristics have been evaluated at different loads and sliding velocities. The microstructural features invariably showed a significant refinement of the primary phases and also modification of secondary phases in Al-alloys. The Pb particles in Al–Si–Pb alloy were observed to be uniformly distributed in the matrix phase besides decorating the grain boundaries. The spray formed composites showed uniform distribution of SiC particles in the matrix. It was observed that wear resistance of Al–Si alloy increases with increase in Pb content; however, there is not much improvement after addition of Pb more than 20%. The coefficient of friction reduced to 0.2 for the alloy containing 20%Pb. A sliding velocity of 1 ms⁻¹ was observed to be optimum for high wear resistance of these materials. Alloying elements such as Fe and Cu in Al–Si alloy lead to improved wear resistance compared to that of the base alloy. The addition of SiC in 2014Al alloy gave rise to considerable improvement in wear resistance but primarily in the low pressure regime. The wear rate seemed to decrease with increase in sliding velocity. The wear response of the materials has been discussed in light of their microstructural features and topographical observation of worn surfaces.     

Investigation of Thermodiffusion Carbonitride Coatings on Titanium Alloys

We study specific features of the processes of phase formation and gas saturation in VT1-0, VT14, OT-4, and VT20 titanium alloys held in a carbon-and-nitrogen-containing medium for 5 h at 1100°C. Depending on the type of alloy, the TiC _x N _y carbonitride phases of different compositions are formed on the metal surface under the indicated conditions. The analysis of the corrosion and electrochemical behavior of alloys shows that the best protective properties are exhibited by the carbonitrides formed on the surface of VT20 alloy (whose composition is close to equiatomic and the surface roughness is minimum). __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 4, pp. 59–65, July–August, 2005.     

Influence of the parameters of nitriding on the subsurface hardening of VT6 titanium alloy

We study the influence of the parameters of nitriding (temperature, time of holding, and pressure of the active gas) on the formation of nitride coatings based on VT16 titanium alloy. We give recommendations concerning the possibility of combination of the prescribed thermal treatment with the thermochemical (nitriding) treatment of the alloy aimed at guaranteeing the required level of subsurface hardening. It is shown that the decrease in the partial pressure of nitrogen to 1–10 Pa increases the depth of the hardened zone and ensures the required level of subsurface hardening. The procedure of heating in a vacuum (1 mPa) performed prior to the action of nitrogen improves the surface quality of the alloy. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 6, pp. 49–54, November–December, 2007.     

Corrosion and stress-corrosion resistance of deposited metals in hydrogen-sulfide solutions

We study the corrosion, electrochemical, and corrosion–mechanical properties of deposited KKh30N6VSR cobalt-based, NKh15SR3 nickel-based, and Kh16N8S5 iron-based alloys in a standard NACE hydrogen-sulfide solution. It is shown that the deposited Kh16N8S5 iron-based alloy is unsuitable for these conditions due to its low corrosion resistance and susceptibility to hydrogen-induced cracking. The cobalt- and nickel-based alloys are suitable for the surfacing of the components of shutoff fittings of the oil-and-gas extraction equipment. However, as far as the “operating characteristics–cost” ratio is concerned, the nickel-based alloy outweighs the cobalt- based alloy.     

Titanium-silicon coatings and their influence on the efficiency of machine parts

The production of titanium-silicon coatings on VT8 titanium alloy and the effect of these coatings on the resistance to contact fatigue and fatigue crack propagation are investigated. It is established that such coatings increase the resistance of VT8 alloy to fatigue crack propagation and contact fatigue.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 79–81, September–October, 1993.     

Changes in the physicomechanical characteristics of VT14 titanium alloy in the process of long-term loading

We study the influence of long-term loading on the behavior of physicomechanical characteristics of VT14 high-strength titanium alloy. The long-term holding of specimens (up to 21 yr) under loads close to the fracture load confirmed the stability of the long-term strength characteristics of the material. Institute of Technical Mechanics, National Academy of Sciences of Ukraine, Dnepropetrovsk, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 144–148, March–April, 2000.     

Influence of microstructure on corrosion behavior of Ti–5%Ta–1.8%Nb alloy

This paper presents the results of a study on the influence of microstructure on the corrosion behavior of a α–β Ti–5%Ta–1.8%Nb alloy—a candidate material for use in high concentrations of boiling nitric acid. The “as cast” alloy had a lamellar structure and showed a corrosion rate of about 1.5 mpy. Thermo-mechanical processing of the cast alloy resulted in a structure of predominantly of equiaxed α with random distribution of fine β particles. This “reference” structure was further modified employing different heat treatments similar to that for commercial titanium alloys such as mill annealing, solution treatment and aging or over aging treatments. Corrosion rates evaluated in boiling nitric acid in the liquid, vapor and condensate phases, showed low values ∼1 mpy. Of these, the lowest corrosion rate (∼0.03 mpy) was exhibited by the structure with minimum amount of β phase, distributed in an equiaxed α matrix. This structure was obtained by aging of the solution treated “reference” alloy. Hence, solution treatment high in the α + β phase field followed by aging at a temperature low in the α + β phase field has been identified as the optimum treatment to obtain a microstructure with superior corrosion resistance.     

Wear characteristics of a new type austenitic stainless steel

In order to solve the problem of the poor wear resistance in conventional austenitic stainless steels, a new type austenitic stainless steel was designed based on Fe–Mn–Si–Cr–Ni shape memory alloys in this article. Studies on its wear resistance and wear mechanism have been carried out by comparison with that of AISI 321 stainless steel using friction wear tests, X-ray diffraction, scanning electron microscope. Results showed that the wear resistance of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was better than that of AISI 321 stainless steel both in dry and oily friction conditions owing to the occurrence of the stress-induced γ → ε martensitic phase transformation during friction process. This article also compared the corrosion performance of the two stainless steels by testing the corrosion rate. Results showed that the corrosion rate of Fe–14Mn–5.5Si–12Cr–5Ni–0.10C alloy was notably lower in NaOH solution and higher in NaCl solution than that of AISI 321 stainless steel.     

Theory of Photothermal Displacement Method for Determining Physical Properties of Multilayered Coatings

In this study theoretical principles underlying the photothermal method for determining thermal properties of opaque multilayered and functionally graded coatings are analyzed. The method is based on irradiation of the assembly by the repetitive pulse of focused laser radiation that is absorbed in the subsurface region and causes non-uniform heating and buckling of a coating. The irradiated surface of a coating is monitored by a low power beam of a second laser that is reflected from the specimen. The deflection angle of the monitoring beam, as a function of time, contains the relaxation and the “wave” components. It is shown that the phase of the “wave” component depends on the thermophysical properties (e.g., thermal diffusivity or thermal conductivity) of a coating. These properties can be determined by comparing experimentally measured values of the phase shift of the “wave” component with the theoretical values obtained from the analytical solution of the two-dimensional thermal elasticity problem for a multilayered coating–substrate assembly.