Microindentation and Inverse Analysis to Characterize Elastic-Plastic Properties for Thermal Sprayed Ti2AlC and NiCoCrAlY

Elastic-plastic material properties for HVOF sprayed Ti₂AlC (sprayed with Maxthal 211 powder) and plasma sprayed NiCoCrAlY coatings were investigated using modeling and experimental Berkovich microindentation. Optical microstructure evaluations were also performed. The theories of Hertz, Oliver and Pharr were combined with finite element analysis for extracting the material properties. Empirically based material models for both thermal sprayed Ti₂AlC and NiCoCrAlY coatings are proposed.     

Effects of TiC and Al4C3 addition on combustion synthesis of Ti2AlC

Preparation of the ternary carbide Ti₂AlC was conducted by combustion synthesis in the mode of self-propagating high-temperature synthesis (SHS) from the elemental powder compacts of Ti:Al:C = 2:1:1, TiC-containing samples with TiC of 6.67–14.3 mol%, and Al₄C₃-containing samples with Al₄C₃ of 1.96–10 mol%. Effects of TiC and Al₄C₃ addition were studied on combustion characteristics and the degree of phase conversion. Due to the growth of laminated Ti₂AlC grains, the reactant compact was subjected to an axial elongation during the SHS process. Because the addition of TiC and Al₄C₃ led to a decrease in the reaction temperature, the flame-front propagation velocity was correspondingly reduced for the TiC- and Al₄C₃-containing samples when compared with the elemental reactants. Based upon the XRD analysis, formation of Ti₂AlC along with a secondary phase TiC was identified in the synthesized products. The grains of Ti₂AlC are typically plate-like with a size of 10–20 μm and several laminated Ti₂AlC grains form a layered structure. The content of Ti₂AlC yielded from the elemental powder compacts is about 85 wt%. The addition of TiC was found to facilitate the formation mechanism and therefore to enhance the extent of Ti₂AlC conversion approaching 90 wt%. As a result of the reduced exothermicity of the reaction, however, the content of Ti₂AlC decreased slightly in the products synthesized from the Al₄C₃-added samples.     

Processing and characterization of porous Ti2AlC with controlled porosity and pore size

In this work, we demonstrate a simple and inexpensive way to fabricate porous Ti₂AlC, one of the best studied materials from the MAX phase family, with controlled porosity and pore size. This was achieved by using NaCl as the pore former, which was dissolved after cold pressing but before pressureless sintering at 1400 °C. Porous Ti₂AlC with samples a volume fraction of porosity ranging from ～10 to ～71 vol.% and different pore size ranges, i.e. 42–83, 77–276 and 167–545 μm, were successfully fabricated. Fabricated samples were systematically characterized to determine their phase composition, morphology and porosity. Room temperature elastic moduli, compressive strength and thermal conductivity were determined as a function of porosity and/or pore size. For comparison, several samples pressureless-sintered without NaCl pore former, or fabricated by spark plasma sintering, were also characterized. The effects of porosity and/or pore size on the room temperature elastic moduli, compressive strength and thermal conductivity of porous Ti₂AlC are reported and discussed in this work. It follows that porosity can be a useful microstructural parameter to tune mechanical and thermal properties of Ti₂AlC.     

Formation of TiAl–Ti2AlC in situ composites by combustion synthesis

Preparation of TiAl–Ti₂AlC in situ composites with a broad range of composition was conducted by self-propagating high-temperature synthesis (SHS) with compressed samples from the mixture of elemental powders. When compared with SHS formation of monolithic TiAl, the addition of carbon particles to the Ti–Al powder mixture enhances the sustainability of the reaction. It was found that no prior heating was required for the samples prepared to produce the composites containing more than 20 mol% Ti₂AlC, in contrast to the need of preheating at 200 °C for single-phase TiAl formation. This is attributed to the fact that formation of Ti₂AlC is more exothermic than that of TiAl. As a result, the combustion temperature and combustion wave velocity increase with the content of Ti₂AlC formed in the TiAl–Ti₂AlC composite, and approach the values associated with formation of single-phase Ti₂AlC when considerable amounts of Ti₂AlC are yielded. The XRD analysis of the end products confirms formation of TiAl–Ti₂AlC in situ composites. Moreover, simultaneous formation of Ti₂AlC promotes the phase evolution of the aluminide compounds. That is, the secondary aluminide phase, Ti₃Al, was no longer detected in the TiAl–matrix composites containing Ti₂AlC of 30 mol% or above.     

Comparison of hardmetal and hard chromium coatings under different tribological conditions

Electroplated hard chromium and thermal spray hardmetal coatings are widely used in a variety of applications for wear protection of component surfaces. The two protective coating types are tested in direct comparison for tribological conditions of dry abrasive wear (Taber Abraser test) and dry oscillating wear load. Oscillating wear tests are carried out both with hardened 100Cr6 steel and alumina balls as counterbody. Different types of hardmetal coatings are imparted. Besides HVOF sprayed coatings also coatings sprayed by an APS gun with axial powder feed are tested. For HVOF spraying besides standard WC/Co(Cr) feedstock also coarse (d₅₀ = 5 μm) and fine carbide feedstock (d₅₀ = 0.8 μm) and ultrafine powders, i.e. 2 μm < d < 12 μm, are considered. Use of ultrafine powders is particularly interesting from the economical point of view, as belt grinding can be sufficient for finishing in many cases. The optimum coating solution for wear protection depends on the specific tribosystem. The choice of feedstock, spraying process, equipment and processing conditions does not only depend on the resultant tribological properties. Therefore simultaneous influence on corrosion protection capability and thermal conductivity might have to be considered.     

Oxidation of Ti2AlC bulk and spray deposited coatings

The oxidation behaviour of Ti₂AlC bulk and high velocity oxy-fuel spray deposited coatings has been investigated for temperatures up to 1200 °C. X-ray diffraction and electron microscopy show that bulk Ti₂AlC forms a continuous layer of α-Al₂O₃ below a layer of TiO₂ at temperatures as low as 700 °C. Oxidation of the Ti₂AlC coatings is more complex, and also involves the phases Ti₃AlC₂, TiC, and Ti_xAl_y, formed during the spraying process. α-Al₂O₃ is observed, however, it is unevenly distributed deep into the material, and does not form a continuous layer essential for good oxidation resistance.     

Fabrication and mechanical properties of Ti2AlC/TiAl composites with co-continuous network structure

Ti₂AlC/TiAl composites with different volume fractions were prepared by hot pressing technology, and their reinforced structural characteristics and mechanical properties were evaluated. The results showed that when the reinforced phase volume fraction of Ti₂AlC was 20%, three-dimensional interpenetrating network structures were formed in the composites. Above 20%, Ti₂AlC phase in the composites accumulated and grew to form thick skeletal networks. The microplastic deformation behavior of Ti₂AlC phase, such as kink band and delamination, improved the fracture toughness of the composites. Comparative analysis indicated that the uniform and small interconnecting network structures could further reinforce the composites. The bending strengths of composites prepared with 20 vol.% Ti₂AlC reached (900.9±45.0) MPa, which was 25.5% higher than that of TiAl matrix. In general, the co-continuous Ti₂AlC/TiAl composite with excellent mechanical properties can be prepared by powder metallurgy method.     

Microstructure and some properties of TiAl-Ti2AlC composites produced by reactive processing

The TiAl–Ti₂AlC composites with and without impurities, Ni, Cl and P, were prepared by combustion reaction from the elemental powders and cast after arc melting. The resulting composites had about 18 vol% Ti₂AlC in the lamellar matrix consisting of γ-TiAl and Ti₃Al (α₂). In the homogenized specimens, the α₂ phase decomposed to γ-TiAl and Ti₂AlC. The composite material had a high strength both at ambient and elevated (1173 K) temperatures; about 800 and 400 MPa, respectively, with an ambient temperature ductility of 0.7% at bending test. The fracture toughness test also proved that the homogenized composite has higher toughness than the as cast one. The toughness value reached to 17.8 MPa m^1/2. The zigzag cracks propagated in the homogenized composite and the reinforcement Ti₂AlC particles and the finely precipitated Ti₂AlC particles were main obstacles to the crack propagation. The composite with impurities showed a marginal improvement in the oxidation resistance over the composites without impurities.     

Corrosion behavior of select MAX phases in NaOH, HCl and H2SO4

In this paper we report on the electrochemical corrosion of select MAX phases, namely Ti₂AlC, (Ti,Nb)₂AlC, V₂AlC, V₂GeC, Cr₂AlC, Ti₂AlN, Ti₄AlN₃, Ti₃SiC₂ and Ti₃GeC₂ in 1 M NaOH, 1 M HCl and 1 M H₂SO₄ solutions. Polarization characteristics recorded in 1 M NaOH show that V₂AlC, V₂GeC and Cr₂AlC undergo active dissolution at potentials more positive than the corrosion potential, while Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate. In the 1 M HCl solutions, Ti₂AlC, V₂AlC and V₂GeC actively dissolve; Ti₃SiC₂ and Ti₃GeC₂ passivate. Depending on potential, (Ti,Nb)₂AlC and Cr₂AlC showed trans-passive behavior. In 1 M H₂SO₄ solutions, Ti₂AlC, (Ti,Nb)₂AlC, Ti₃SiC₂ and Ti₃GeC₂ passivate, V₂AlC and V₂GeC show active dissolution, while Cr₂AlC exhibits trans-passive behavior. Ti₂AlN and Ti₄AlN₃ were passive in all solutions except in 1 M HCl, where Ti₂AlN showed trans-passive behavior. Given that the corrosion behavior of (Ti,Nb)₂AlC is unlike either Ti or Nb, the behavior of the former cannot be predicted from that of the latter.     

Oxygen incorporation in M2AlC (M = Ti,V, Cr)

Oxygen incorporation in the MAX phases Ti₂AlC, V₂AlC and Cr₂AlC was studied by ab initio calculations. Comparing the calculated energies for oxygen incorporation indicates that oxygen substitutes for carbon in Ti₂AlC and V₂AlC, but is incorporated interstitially in the Al layer of Cr₂AlC, even for carbon-deficient Cr₂AlC. To evaluate these predictions, combinatorial d.c. sputtering was used to deposit thin films with different oxygen concentrations. Two phase regions of Cr₂AlC and Cr₂Al were investigated in order to study oxygen incorporation in carbon-deficient Cr₂AlC. X-ray strain analysis data indicate that the a and c lattice parameters increase with oxygen content. These trends are in good agreement with the change in lattice parameters predicted by ab initio calculations and therefore corroborate the notion of interstitial oxygen incorporation in Cr₂AlC. A metastable solubility limit for oxygen of 3.5 at.% was determined experimentally. This is the first report on interstitial oxygen in a MAX phase and may be of relevance during the initial stages of oxidation.     

Comparative investigation of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti2 − хCrхAlC targets

A comparative investigation of the structure and properties of TiAlC(N), TiCrAlC(N), and CrAlC(N) coatings deposited by sputtering of МАХ-phase Ti_2 − хCr_хAlC targets (where x = 0, 0.5, 1.5, and 2) in an Ar atmosphere or in a gaseous mixture of Ar + N₂ is presented. The coatings were characterized in terms of their structure, elemental and phase composition, hardness, elastic modulus, elastic recovery, thermal stability, friction coefficient, wear rate, corrosion, and high-temperature oxidation resistance. The structure of the coatings was studied by means of X-ray diffraction, scanning and transmission electron microscopy, X-ray photoelectron spectroscopy, glow discharge optical emission spectroscopy, electron energy loss spectroscopy, and Raman spectroscopy. To evaluate the thermal stability and oxidation resistance, the coatings were annealed either in vacuum or in air at temperatures 600-1200 °C. The results obtained show that the TiAlCN coatings possess high hardness of 32-35 GPa, low friction coefficient against WC-Co well below 0.25, high thermal stability up to 1200 °C, and superior performance in dry milling tests against high Cr steel. Meanwhile, the coatings with high Cr content demonstrated improved oxidation resistance up to 1000 °C and superior electrochemical behavior, but their mechanical and tribological properties were deteriorated.     

Preparation of Ti3AlC2 by mechanically activated sintering with Sn aids

The mechanically activated sintering process was adapted to synthesize Ti₃AlC₂ using 3Ti/Al/2C/0.05Sn powder mixtures. The result showed that the powders containing TiC, Ti₃AlC₂ and Ti₂AlC were obtained by mechanical alloying (MA) 3Ti/Al/2C powders. Addition of appropriate Sn reduced the content of Ti₂AlC and enhanced the synthesis of Ti₃AlC₂ significantly. The powders with highest content of Ti₃AlC₂ were obtained by MA 3Ti/Al/2C/0.05Sn powders. Through pressureless sintering the mechanical alloyed powders at 900–1100 °C for 2 h, the high purity Ti₃AlC₂ material with fine organization was produced.     

Formation Routes of Nanocomposite Coatings in Detonation Spraying of Ti3SiC2-Cu Powders

In thermally sprayed coatings, nano-sized features of the microstructure may be either inherited from the nanostructured agglomerates of the feedstock powder or form as a result of rapid cooling of molten particles upon deposition. Applying a process of the computer-controlled detonation spraying (CCDS) to Ti₃SiC₂-Cu composite powders produced by high-energy mechanical milling, we show that both routes are possible depending on the spraying conditions. When the nanostructure of the Ti₃SiC₂-Cu coating is inherited from the feedstock powder—under very mild conditions of detonation spraying, which exclude melting, so is the phase composition of the coating. In higher-temperature conditions of spraying, a significant fraction of the copper matrix melts and the interaction between Ti₃SiC₂ and Cu occurs. The TiC _x -Cu(Si) coatings that form show crystallites of both phases in the nano-range. In this case, rapid solidification of the molten fraction of the particles is responsible for the formation of the coatings with a nanostructured matrix. Due to the flexibility of the CCDS process, conditions of spraying were found such that a composite coating with very fine crystallites of the Cu(Si) matrix (30 nm) and a hardness of 273 HV could be obtained.     

Formation of TiC/Ti2AlC and α2+γ in in-situ TiAl composites with different solidification paths

In order to improve the mechanical properties of TiAl alloys, TiAl composites with different solidification paths were synthesized by metallurgical method. Results show the TiC disappears and Ti₂AlC increases when the Al content is more than 42% (at.%, similarly hereinafter). Small TiC particles are located in Ti₂AlC grains with irregular shapes when the Al content is 40%, and they translate into clubbed Ti₂AlC with increasing of Al. This metallurgy method can solve the defects of the Al lacking and the residual TiC. The γ phase increases between lamellar colonies with the increasing of Al. When the Al content is 48%, the fully lamellar structure transforms into a duplex microstructure and there are small Ti₂AlC phases in γ phases, because the forming of Ti₂AlC phase must consume Al. The compressive strength increases up to 1678.68 MPa as Al content is 46 at.%, and then decrease to 1460.22 MPa, the compressive strain increases and then keeps stabilization with the increasing Al. The maximum strength improves 38.82% and the maximum strain improves 121.37%. The Ti₂AlC/TiAl composites fracture behaviors are load transferring behavior, crack deflection, trans-lamellar cracking and extraction of carbide reinforcements. The Ti₂AlC phase and the fully lamellar structure improve the mechanical properties.     

Yttria stabilized zirconia corrosion destabilization followed by Raman mapping

The incidence of V₂O₅ corrosion on yttria stabilized zirconia (YSZ) thermal barrier coatings has been studied as a function of application methods and powder initial granulometry. Commercial fused-and-crushed 8 wt.% yttria-stabilized zirconia was sprayed by Atmospheric Plasma Spraying (APS) and High Frequency Pulse Detonation. Hollow Spherical Powder (HOSP™) with the same composition was sprayed by APS. The coatings where covered with V₂O₅ powder and treated at 1000 °C for different times. The extent of corrosion was followed by X-ray diffraction, scanning electron microscopy and Raman micro-spectrometry. A relationship between coating porosity and corrosion resistance is explored: the HOSP coating presented deeper penetration of corrosion than the other coatings. The authors present the extended capabilities of Raman semi-quantitative phase analysis to describe the depth and density of yttria leaching by vanadia leading to YSZ destabilization.     

Microstructural characterization and hardness evaluation of HVOF sprayed Ni–5Al coatings on Ni- and Fe-based superalloys

HVOF sprayed Ni–5Al coatings on Ni- and Fe-based superalloy substrates were characterized to assess the microstructural features and strength in the as deposition condition for their applications in high-temperature corrosive environment of gas turbine. X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), and X-ray mapping analysis are used to characterize the Ni–5Al coatings. The dense coatings with less porosity and inclusions were produced using HVOF process. The deposited Ni–5Al coatings exhibited splat like layered morphologies due to deposition and resolidification of successive molten and semi molten powder particles. The hardness of coatings on three different superalloy substrates was measured and it was in the range of 210–272 Hv. The average bond strength and surface roughness of the as-sprayed coatings were 42.62 MPa and 9.22–9.45 μm, respectively. Diffusion of alloying elements from the substrate into the coating has occurred in all the three superalloy substrates as observed from the X-ray mapping analysis.     

Thermoelectric properties of Ti1+xS2 prepared by CS2 sulfurization

Titanium disulfide (TiS₂) powder was prepared by sulfurizing TiO₂ powder with CS₂ gas at 1073 K for 4 h. Because CS₂ gas is a powerful sulfurizing agent for TiO₂, CS₂ sulfurization can be performed at a low temperature. The TiS₂ powder thus prepared was first mixed with sulfur powder in a mass ratio of 1:0.04 and pressure sintered at 973 K for 1 h under a uniaxial pressure of 50 MPa in vacuum. The addition of a small amount of sulfur powder to the TiS₂ powder prevents sulfur deficiency in the sintered compact, resulting in the formation of a near-stoichiometric Ti_1.008S₂ composition. X-ray diffraction patterns show that the crystalline ab-axis is preferentially oriented perpendicular to the pressing direction. The Seebeck coefficient, electrical resistivity and thermal conductivity of the oriented TiS₂ sintered compacts with near-stoichiometric and sulfur-poor (titanium-rich) compositions were measured in the temperature range 300–723 K. The thermoelectric figure of merit ZT was enhanced by prevention of sulfur deficiency and formation of the oriented texture. The highest ZT of 0.34 was observed at 663 K in Ti_1.008S₂ for the direction perpendicular to the pressing axis.     

Comparative studies on corrosion and tribological performance of multilayer hard coatings grown on WC-Co hardmetals

Multilayer TiN/TiCN/TiCN/TiC/TiN and TiN/TiCN/TiCN/TiC/Al₂O₃ hard coatings with total thicknesses of 15.7 μm and 9.3 μm were deposited on WC-10Co substrates using a chemical vapor deposition system. Evaluation of surface, cross-section morphologies, chemical composition and phases of coatings were analyzed by field emission scanning electron microscopy (FESEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD) analyses respectively. Corrosion properties were evaluated in 3.5 wt% NaCl medium using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). Tribological properties of fabricated multilayer hard coatings were evaluated using pin-on-disk tests. Results show that active dissolution of the WC-10Co occurred while the coated samples showed more anodic slopes as well as lower corrosion current densities. The corrosion current densities of 3.3 × 10⁻⁷ A/cm² and 7.5 × 10⁻⁸ A/cm² were obtained for the TiN/TiCN/TiCN/TiC/TiN and TiN/TiCN/TiCN/TiC/Al₂O₃ coated specimens which are much lower than 4 × 10⁻⁶ A/cm² of substrate. EIS analysis confirmed the results of potentiodynamic polarization curves. Delamination of the TiN coating and formation of titanium oxide compounds on the surface of the TiN/TiCN/TiCN/TiC/TiN coating revealed that oxidative wear mechanism is dominant for this sample, while adhesive wear mechanism was dominant for the TiN/TiCN/TiCN/TiC/Al₂O₃ coated sample.     

Microstructure and shape memory behavior of Ti55.5Ni44.5−xCux (x = 11.8–23.5) thin films

The microstructure and shape memory behavior of Ti_55.5Ni_45.5−xCu_x (x = 11.8–23.5) thin films annealed at 773, 873, and 973 K for 1 h were investigated. None of the films except the Ti_55.4Ni_32.8Cu_11.8 film annealed at 773 K for 1 h had any precipitates in the B2 grain interiors and their grain sizes were small (less than 1 μm). Increasing the annealing temperature caused grain growth and thus a decrease in the critical stress for slip and an increase in the martensitic transformation start temperature (Ms). The grain size was also controlled by the growth of a second phase. In the three-phase equilibrium region of Ti₂Ni, Ti₂Cu and TiNi, Ti₂Cu grains grew faster than Ti₂Ni grains, leading to a decrease in the critical stress for slip and an increase in the Ms temperature with increasing Cu content.     

Comprehensive process maps to synthesize high density plasma sprayed aluminum oxide composite coatings with varying carbon nanotube content

Comprehensive process maps have been developed to synthesize “high density” plasma sprayed Aluminum oxide (Al₂O₃) composite coatings with 0, 4 and 8 wt.% carbon nanotube (CNT) reinforcement. These process maps correlate the key processing parameters such as plasma power, powder feed rate, primary gas flow rate, and stand-off distance to the temperature and velocity of the in-flight particles and finally, to the porosity and microhardness of coatings. Relative importance of each significant process parameters was also investigated by making “Pareto diagrams”. Stand-off distance followed by the plasma power were found to be most sensitive parameters which affects the porosity of the plasma sprayed Al₂O₃-CNT composite coatings. Process maps showed that CNTs have a significant effect on altering the temperature and velocity of in-flight particles. Higher thermal conductivity of CNTs alters the heat transfer phenomena during the coating build up which assists in densification.