Microstructure, microhardness and dry friction behavior of cold-sprayed tin bronze coatings

In this paper, two types of tin bronze coatings (Cu-6 wt.% Sn and Cu-8 wt.% Sn) were prepared by cold spray process. The as-sprayed coatings were subjected to a vacuum heat treatment at 600 °C for 3 h. The coating microstructure, microhardness and tribological performance were characterized. The effects of the tin content and the vacuum heat treatment on the microstructure, microhardness and tribological behavior of the coatings were investigated. It is found that the as-sprayed CuSn6 (As6) and CuSn8 (As8) coatings exhibit practically an identical porosity. Meanwhile, As8 presents a higher microhardness than As6. In addition, the increase of the tin content in the powder feedstock leads to a lower wear rate. After a heat treatment, coating porosities are significantly reduced. However, the coating hardness is significantly decreased and the coating presents a much decreased wear resistance. For the as-sprayed coatings, such factors as ploughing and particle delamination could determine the sliding process. The heat treatment results in a distinct modification of the tribological behavior. For the annealed coatings, the adhesion, between the coating and the counterpart, could play a dominant role in the sliding process.     

Effect of impact-induced melting on interface microstructure and bonding of cold-sprayed zinc coating

Zn particles are employed to create different impact conditions, including impact-induced interface melting in cold spraying. The influence of particle impact conditions on the interfacial microstructure evolution, microhardness and the bonding of particles in cold-sprayed Zn coatings are studied. An examination of coating surface morphology provides convincing evidence for melting at particle interfaces. The results reveal that the nanostructured phase was formed at the interface areas between deposited particles in coating resulting from the recrystallization of deformed grains. Melting at interfaces significantly enhances the bonding between the substrate and the coating and between the deposited Zn particles in the coating through the formation of a metallurgical bond. In addition, high driving gas temperature causes the decreasing hardness of deposited Zn coatings. The effects of particle conditions on the impact-induced melting and bonding mechanisms are discussed.     

Characteristics and heat treatment of cold-sprayed Al-Sn binary alloy coatings

In this study, Al-Sn binary alloy coatings were prepared with Al-5 wt.% Sn (Al-5Sn) and Al-10 wt.% Sn (Al-10Sn) gas atomized powders by low pressure and high pressure cold spray process. The microstructure and microhardness of the coatings were characterized. To understand the coarsening of tin in the coating, the as-sprayed coatings were annealed at 150, 200, 250 and 300 °C for 1 h, respectively. The effect of annealing on microstructure and the bond strength of the coatings were investigated. The results show that Al-5Sn coating can be deposited by high pressure cold spray with nitrogen while Al-10Sn can only be deposited by low pressure cold spray with helium gas. Both Al-5Sn and Al-10Sn coatings present dense structures. The fraction of Sn in as-sprayed coatings is consistent with that in feed stock powders. The coarsening and/or migration of Sn phase in the coatings were observed when the annealing temperature exceeds 200 °C. Furthermore, the microhardness of the coatings decreased significantly at the annealing temperature of 250 °C. EDXA analysis shows that the heat treatment has no significant effect on fraction of Sn phase in Al-5Sn coatings. Bonding strength of as-sprayed Al-10Sn coating is slightly higher than that of Al-5Sn coating. Annealing at 200 °C can increase the bonding strength of Al-5Sn coatings.     

Effect of vacuum heat treatment on tensile strength and fracture performance of cold-sprayed Cu-4Cr-2Nb coatings

The previous study [1] indicated that dense thick Cu-4Cr-2Nb coatings could be formed by cold spraying, and the post-spray heat treatment could significantly influence the microstructure and microhardness of the as-sprayed Cu-4Cr-2Nb coatings. In this study, the tensile strength and fracture performance of the Cu-4Cr-2Nb coatings after annealing were investigated. The vacuum heat treatment was conducted under 10⁻² Pa at 850 °C for 4 h. Results showed that the heat treatment had a great contribution to the healing-up of the incompleteness of the interfaces between the deposited particles. In addition, the coating microhardness decreased from 156.8 ± 4.6 Hv_0.2 for the as-sprayed coatings to 101.7 ± 4.5 Hv_0.2 for the annealed ones. The mean tensile strength of the annealed coatings was approximately 294.1 ± 36.1 MPa compared to that of 45.0 ± 10.5 MPa for the as-sprayed ones, which results from the partially metallurgically bonded zones between the deposited particles inducing by the heat treatment process.     

Characterization of cold-sprayed nanostructured Fe-based alloy

The ball-milled Fe-Si alloy was used as feedstock for deposition of nanocrystalline Fe-Si by cold spraying process. The microstructure of the as-sprayed nanostructured Fe-Si was characterized by using optical microscopy, scanning electron microscopy and transmission electron microscopy. The grain sizes of the feedstock and as-sprayed deposit were estimated based on X-ray diffraction analysis. The microhardness and coercivity of the deposited Fe-Si alloy were characterized. The results showed that the as-sprayed deposit presented a dense microstructure. The mean grain size of the as-deposited Fe-Si was several tens nanometers and comparable to that of the corresponding milled feedstock. The temperature of driving gas presented little effect on the microstructure of cold-sprayed nanostructured Fe-Si deposit. The mechanical alloying induced oxygen contents up to 8 wt% in the feedstocks and subsequent deposits. The microhardness of the deposit reached about 400 Hv. The deposit achieved a high coercivity up to 190 kA/m indicating the potential possibility for applications to recording materials.     

Influence of annealing treatment on the microstructure and mechanical performance of cold sprayed 304 stainless steel coating

In this study, 304 stainless steel coatings were deposited on interstitial-free steel substrate by cold spraying method. The effect of annealing treatment on microstructure, microhardness, ultimate tensile strength and fracture performance of the coatings were studied. The results showed that annealing treatment had made a dominant contribution to heal up the incomplete interfaces between the deposited particles. Both of the microstructure and the mechanical properties have been obviously optimized by annealing treatment. In addition, the coating microhardness decreased from 345 HV_0.2 for the as-sprayed coating to 201 HV_0.2 for the annealed coating. The coating ultimate tensile strength increased from 65 MPa for the as-sprayed coating to 357 MPa for the annealed coating, which resulted from the increase of the metallurgically bonded areas in the coating induced by annealing treatment. Fracture morphology of the coatings also revealed that annealing treatment changed the fracture character of the cold sprayed 304 stainless steel coating from brittle type to plastic type.     

Effects of Ta on microstructure and microhardness of Ni based laser clad coating

Through addition of Tantalum, fine TaC particles were in situ synthesized in a NiCrBSi alloy laser clad composite coating. Microstructure, microhardness and abrasive wear resistance of the composite coating were investigated. The result showed that TaC particles were dispersed in Ni based alloy composite coating, refining the microstructure of the coating after laser cladding. Amount of coarse primary carbides such as M7C3 and eutectic of γ-Ni + M23C6 substantially decreased because the formation of TaC particles suppressed the formation of M7C3 and M23C6. On the one hand, fine TaC particles acted as hard phase, which improved the microhardness of the composite coating; on the other hand, a decrease in amount of the coarse M7C3 and eutectic of γ-Ni + M23C6 reduced the crack susceptibility of the Ni based composite coating. Also, Ta element improved the abrasive wear resistance of the Ni based coating.     

Significant influence of particle surface oxidation on deposition efficiency, interface microstructure and adhesive strength of cold-sprayed copper coatings

The critical velocity for particle deposition in cold spraying is a key parameter, which depends not only on the material type, but also the particle temperature and oxidation condition. The dependency of deposition efficiency of cold spray Cu particles on the particle temperature and surface oxidation was examined. The effect of particle surface oxide scales on the interfacial microstructure and adhesive strength of the cold-sprayed Cu coatings was investigated. The results show that the deposition efficiency significantly increases with increasing the gas temperature but decreases with augmenting the oxygen content of the starting powder. The oxide inclusions at the interfaces between the deposited particles inhibit the effective bonding of fresh metals and remarkably lower the bond strength of the deposited Cu coatings on steel.     

Effect of the heat treatment on the infrared emissivity of indium tin oxide (ITO) films

Indium tin oxide (ITO) films were deposited on glass substrates at temperatures ranging from 100 °C to 400 °C by direct current magnetron sputtering. The mean infrared emissivities at the waveband of 8-14 μm were measured in process of heating and cooling between room temperature and 350 °C. Microstructure and phases of ITO films before (Group A) and after (Group B) heat treatment were characterized by SEM and XRD, respectively. Electrical properties were characterized with a four-point probe method and by Hall measurement system. During heat treatment, the infrared emissivity of the film increases with the increase of temperature, and decreases with the decrease of temperature. While, the infrared emissivity of the films decreases slightly around 250 °C in heating process. On the other hand, after heat treatment, the crystalline phases of the films have no obvious change. However, both the resistivity and the infrared emissivity of all films decrease.     

The effect of heat treatment on the electrochemical corrosion behavior of reactive plasma-sprayed TiN coatings

The effect of heat treatment on the corrosion behavior of reactive plasma sprayed TiN coatings in simulated seawater was investigated by electrochemical methods such as the corrosion potential-time curve (E_corr − t), potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and SEM, etc. The results showed that the corrosion potential of TiN coatings increased after heat treatment; the corrosion current of the TiN coatings after heat treatment (be hereafter referred to as HT-TiN) was 13.3% of the untreated coatings (be hereafter referred to as UT-TiN), and the polarization resistance of HT-TiN was 20 times of UT-TiN, which indicated that the heat treatment had significantly increased the corrosion resistance of the coatings. The corrosion behavior of the coatings was mainly local corrosion, and the local corrosion behavior mainly took place at the microdefects (crack and pores) of the coatings. The porosity of the coatings was reduced after heat treatment. The reason was that TiN reacted with O₂ to form TiO₂ and Ti₃O during the heat treating, and volume expansion took place, which led to denser microstructure. The corrosion resistance of the coatings was therefore increased.     

Effect of the heat treatment conditions on the synthesis of Sr-hexaferrite

The effect of heat treatment conditions under oxygen atmosphere on the SrFe₁₂O₁₉ synthesis is analyzed. Effect of partial evacuation of decomposition gases of the organometallic precursor on the phase composition of different samples is studied. An accurate structural analysis of samples obtained between 250 °C and 600 °C is reported. From the structural analysis several secondary phases are identified. The amount of secondary phases can be manipulated through the control of the heat treatment conditions, and therefore, this constitutes a methodology to manipulate the composition and the magnetic properties of the obtained nanopowders. The quantitative determination of phases is performed by structural refinement of X-ray powder patterns, using Rietveld analysis. Magnetic study is done by magnetization vs. applied magnetic field at room temperature.     

Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.     

Investigation of the microstructure and tribological behavior of cold-sprayed tin-bronze-based composite coatings

In this paper, tin-bronze/TiN and tin-bronze/quasicrystal (AlCuFeB) composite coatings were fabricated by cold spray process. Microstructure and microhardness of the prepared coatings were investigated. Ball-on-disc dry sliding wear tests were conducted in an ambient condition to examine the tribological behavior of the composite coatings. The results show that the microhardness and the density of composite coatings increase significantly compared to the pure tin-bronze coating. The friction coefficient of composite coating decreases when reinforcing particles were introduced. Furthermore, the bronze/quasicrystal composite coating has a lower friction coefficient and wear rate than the bronze/TiN coating. Tribological mechanisms of the composite coatings were discussed.     

Deposition characteristics of Al-12Si alloy coating fabricated by cold spraying with relatively large powder particles

In this paper, the microstructure, microhardness and adhesive strength of Al-12Si coating produced by cold spraying were investigated. It is found that a thick, dense and well bonded Al-12Si coating could be produced by cold spraying with a relatively large powder through the control of spray conditions. The critical velocity for large Al-12Si particles was lower than that of small Al-12Si particles. The as-deposited Al-12Si coating had the same crystal structure as Al-12Si powder. The localized interface melting occurred resulting from both the adiabatic shearing upon impact and the thermal effect of hot gas. Some fine Si particles precipitated in α-Al matrix because of the thermal effect of hot gas during coating deposition. The dispersed Si particles in Al-12Si coating improved the coating microhardness.     

Study on impact fusion at particle interfaces and its effect on coating microstructure in cold spraying

This paper deals with the impact melting phenomenon at the interfaces between the deposited particles in cold-sprayed coatings and its effect on coating microstructure and particle bonding mechanism. Al-12Si, Al2319, Ti, Ti-6Al-4V, Ni and NiCoCrAlTaY powders were selected as feedstocks, which have various thermal and mechanical properties. The analytical results showed that most of the used materials possibly experienced the local melting at the contact interfaces of particles under certain impact conditions. Low melting point, relatively high gas temperature and chemical reaction with the atmosphere are the main factors contributing to the impact fusion during cold spraying. The results also indicated that the local melting would benefit the formation of a metallurgical bonding between the deposited particles and enhance the coating cohesion.     

Effect of heat treatment on bioactivity of anodic titania films

Anodic oxidation could be employed to produce crystalline titania films on Ti6Al4 V surfaces for inducing apatite formation in simulated body fluid (SBF). In this work, the effect of further heat treatment on the bioactivity of anodic titania films was researched. The surface constitution, morphology, crystal structure and apatite-forming ability of titania films were characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated the apatite formation on the Ti6Al4 V surfaces could be attributed to abundance of Ti-OH groups formed via anodic oxidation, but subsequent heat treatment would decrease the amount of surface hydroxyl (OH) groups and result in the loss of the apatite-forming ability.     

Experimental studies on the microstructure and hardness of laser-treated steel specimens

An experimental investigation with 5 kW CW CO₂ laser system was carried out to study the effects of different laser and process parameters on the microstructure and hardness of carbon steel specimen with varying carbon percentage. The laser beam is allowed to scan on the surface of the work piece varying the power (1.1–2.5 kW) and traverse speed (6–15 mm/s) at two different spot sizes using TEM_01* mode laser beam. The most hardenable microstructure achieved in case of three grades of carbon steel and the most influencing parameter on the value of hardness are reported. Besides the above study, some multipass operations are also carried out to recommend an appropriate gap between consecutive passes.     

The bond strength of Al-Si coating on mild steel by kinetic spraying deposition

Kinetic spraying (or cold gas dynamic spraying) works by accelerating small solid particles to supersonic velocities, and then impacting them onto a substrate. These high impact velocities, and low particle temperatures are the principal attributes of kinetic spraying technology. However, only recently has this technology's interfacial behavior, due to particle/substrate impaction, become well understood. In order to investigate the particle/substrate bond behavior, Al-Si feedstock was deposited onto mild steel, over a range of particle velocities; next, their respective coating bond strengths were measured by the stud pull coating adherence test. The effects of the particle velocity and the substrate surface roughness on the coating bond strength were presented, and a model of the particle/substrate bond generation was discussed in an effort to estimate the bond strength.     

Heat treatment effects on microstructure and magnetic properties of Mn-Zn ferrite powders

Mn-Zn ferrite powders (Mn_0.5Zn_0.5Fe₂O₄) were prepared by the nitrate-citrate auto-combustion method and subsequently annealed in air or argon. The effects of heat treatment temperature on crystalline phases formation, microstructure and magnetic properties of Mn-Zn ferrite were investigated by X-ray diffraction, thermogravimetric and differential thermal analysis, scanning electron microscopy and vibrating sample magnetometer. Ferrites decomposed to Fe₂O₃ and Mn₂O₃ after annealing above 550 °C in air, and had poor magnetic properties. However, Fe₂O₃ and Mn₂O₃ were dissolved after ferrites annealing above 1100 °C. Moreover, the 1200 °C annealed sample showed pure ferrite phase, larger saturation magnetization (M_s=48.15 emu g⁻¹) and lower coercivity (H_c=51 Oe) compared with the auto-combusted ferrite powder (M_s=44.32 emu g⁻¹, H_c=70 Oe). The 600 °C air annealed sample had the largest saturation magnetization (M_s=56.37 emu g⁻¹) and the lowest coercivity (H_c=32 Oe) due to the presence of pure ferrite spinel phase, its microstructure and crystalline size.     

One way of surface alloying treatment on iron surface based on surface mechanical attrition treatment and heat treatment

A method of surface alloying treatment has been developed: Ni powders were welded into the surface of iron plates by Surface Mechanical Attrition Treatment (SMAT), followed by annealing at certain temperature for 30 min. A Ni-Fe alloy layer with thickness about 100 μm in the sample surface was fabricated on pure iron plate. Scanning electron microscope (SEM), glow discharge spectrum (GDS), and X-ray diffraction (XRD) methods were used to analyze the microstructure, the composition and the phases of the alloy layer. Studies on the interface microstructure indicated that there was significant atomic diffusion and formation of multilayer of intermetallic compound and solid solution in SMAT process. Subsequent annealing accelerates the alloying process. The corrosion test shows the sample by SMAT treated with Ni powders exhibit the best corrosion resistance.