Effect of high dilution on the in situ synthesis of Ni–Zr/Zr–Si(B,C) reinforced composite coating on zirconium alloy substrate by laser cladding

High dilution of transition metals was employed as a new idea for in situ synthesis of Ni–Zr/Zr–Si(B, C) reinforced composite coatings by high power diode laser (HPDL) cladding Ni–Cr–B–Si powders on zirconium substrate. Microstructure, phase composition, the mechanism of in situ synthesis reinforcement and the microhardness of coatings were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and micro-sclerometer. The results reveal that the morphologies and phase constituents are related to the content of alloying elements in powders. In low alloy coatings, the matrix was mainly composed of intermetallic compounds including NiZr and Ni₁₀Zr₇, while the reinforcements consisted of Zr₅Si₄, β-ZiSi, α-ZrSi and ZrC. At the top of high alloy coatings, the matrix was partially comprised of Zr-based amorphous phase with the reinforcements containing ZrB₂. It is thermodynamically favorable for ZrB₂ ceramic reinforcement to form compared to ZrC phase. The microstructure evolution was dependent on the contribution of the high dilution zirconium alloy substrate to the in situ reinforcement synthesis. The microhardness of the coating showed clear improvement compared with zirconium alloy substrate, although high variability was also found.     

Spark plasma sintering of ZrB<Subscript>2</Subscript>–ZrC powder mixtures synthesized by MA-SHS in air

Mechanical activation-assisted self-propagating high-temperature synthesis (MA-SHS) in air was successfully applied to the synthesis of the powder mixtures of ZrB₂ and ZrC as a precursor of the ZrB₂–ZrC composite. When the powder mixtures of Zr/B/C = 4/2/3–6/10/1 in molar ratio were mechanically activated (MA) by ball milling for 45–60 min and then exposed to air, they self-ignited spontaneously and the self-propagating high-temperature synthesis (SHS) was occurred to form ZrB₂ and ZrC. The ZrB₂–ZrC composites were produced from these MA-SHS powders by spark plasma sintering (SPS) at 1800 °C for 5–10 min and showed the fine and homogeneous microstructure composed of the <5 μm-sized grains. The mechanical properties of the composites evaluated by Vickers indentation method showed the values of Vickers hardness of 13.6–17.8 GPa and fracture toughness of 2.9–5.1 MPa·m^1/2, depending on the molar ratio of ZrB₂/ZrC. Thus, the better microstructure and mechanical properties of the ZrB₂–ZrC composites were obtained from the MA-SHS powder mixtures, compared with those obtained from the MA powder, the mixing powder and the commercial powder mixtures.     

Wettability and reactivity between B4C and molten Zr55Cu30Al10Ni5 metallic glass alloy

The isothermal wetting and spreading behaviors of molten Zr₅₅Cu₃₀Al₁₀Ni₅ metallic glass alloy on B₄C substrates were studied using a modified sessile drop method at 1133–1253 K in a high vacuum. A distinct reaction layer consisting of ZrB₂ and ZrC_x was produced at the interface and displayed good wettability with the molten alloy. The entire spreading kinetics could be characterized by four representative stages: (i) an initial rapid spreading presumably driven by adsorption of the active Zr atoms at the solid–liquid interface, (ii) a quasi-linear and (iii) a linear spreading stage controlled by the chemical reaction between Zr and B₄C in both cases, and (iv) an approach-to-equilibrium stage with precipitation of crystals in the liquid. An increase in temperature promotes the wetting and reaction. In view of the reasonable wettability and reactivity, there is a potential for preparing Zr-base bulk metallic glass matrix composites reinforced by in situ ZrC–ZrB₂ hybrid ceramic particulates using B₄C as a reaction agent by way of an infiltration synthesis technique.     

Influence of pyrocarbon amount in C/C preform on the microstructure and properties of C/ZrC composites prepared via reactive melt infiltration

C/ZrC composites were prepared via reactive melt infiltration with zirconium from porous C/C preforms with various pyrocarbon contents. As the pyrocarbon amount in C/C preform increased from 34.1 vol.% to 61.7 vol.%, the densification of C/ZrC composites was hindered and the ZrC content in C/ZrC composites decreased gradually from 35.3 vol.% to 6.3 vol.%. Meanwhile, the flexural strength of C/ZrC composites decreased initially and then increased, but the flexural modulus rose continuously. The flexural strength and modulus of the composites fabricated from the preform with 34.1 vol.% pyrocarbon matrix were 181 ± 4 MPa and 13.0 ± 1.2 GPa, respectively, and the mass loss rate and linear recession rate were 0.0031 g/s and 0.0012 mm/s, respectively.     

Structural changes of boron carbide induced by Zr incorporation

Zr doped boron carbide (B_4.3C) semiconductor was prepared by hot pressing of mixture of boron carbide powder (B_4.3C) and Zr nanocrystals (0.5 at%), to investigate influence of impurity incorporation on the subtle structure of B₄C crystals. XRD analyses indicated that the hot-pressed sample was composed of B_4.3C, (BN)4H, and (ZrB₂3H. Zr introduction does have modified the B_4.3C structure. Especially, remarkable vacancies were led into on the B(3) sites of the C—B—C chain centre. The effect of Zr incorporation seems to be unique because similar structural change was not observed by the same experimental procedure with Ni doping. XPS studies revealed that the Zr atoms existed in a state with unsaturated bonding. B_4.3C with interstitial Zr atoms is speculated.     

An antiemission coating based on zirconium carbide

The main requirement for the formation of an antiemission coating of intermetallic Pt₃Zr compound is the presence of a buffer layer of stoichiometric zirconium carbide (ZrC) that can be formed with the aid a vacuum-arc plasma source. It is shown that ZrC layer can be obtained through vacuum annealing of a multilayer film comprising nanolayers of zirconium (Zr), nonstoichiometric zirconium carbide (ZrC_{1 ? x} ), and zirconium carbide with excess carbon (ZrC + C) sequentially deposited from vacuum-arc-discharge plasma.     

Effects of Cf/C density on microstructure and properties of 3-D Cf/ZrC composites prepared by liquid metal infiltration

Three-dimensional braided carbon fiber-reinforced ZrC matrix composites, 3-D C_f/ZrC, were fabricated by Liquid metal infiltration process at 1200 °C. Porous carbon/carbon (C_f/C) composites with various densities were used as preforms, and the effects of C_f/C density on microstructure and properties of the 3-D C_f/ZrC composites were investigated. The results show that the composites are composed of carbon, ZrC and residual metal. Both microstructure and properties of the 3-D C_f/ZrC composites are apparently affected by C_f/C density. With increasing density of C_f/C preform, the density of 3-D C_f/ZrC composites decreases while the open porosity increases. The composites obtained from the C_f/C preform with a density of 1.12 g/cm³ have the best mechanical properties, with flexural strength of 286.2 ± 11.4 MPa, elastic modulus of 83.5 ± 6.8 GPa and fracture toughness of 9.2 ± 0.6 MPa m^1/2. The composites exhibit excellent ablation resistance, and the mass rate and the linear ablation rate under an oxyacetylene torch are as low as 5.1 ± 0.4 mg s⁻¹ and 1.1 ± 0.3 μm s⁻¹, respectively.     

Synthesis of Fe based ZrB2 composite coating by gas tungsten arc welding

Abstract

ZrB₂/Fe composite coating was in situ synthesised by gas tungsten arc welding cladding process on AISI 1020 steel. Zr, B₄C and Fe–B alloy powders were used as precursor powders. The phase composition and microstructure were investigated by X-ray diffraction analysis, optical microscopy, scanning electron microscopy and energy dispersive spectroscopy. Microhardness of ZrB₂/Fe composite coating at room temperature was examined. Main phases obtained from Zr and B₄C precursor are ZrB₂ and α-Fe, and those obtained from Zr and Fe–B precursor are ZrB₂ and FeB. In the upper part of these composite coatings, ZrB₂ phase mainly grows along temperature gradient direction. The middle part of these composite coatings has the highest ZrB₂ content and highest microhardness. Gradient dispersions of ZrB₂ reinforcements appeared in the composite coating from the middle to the bottom, leading to gradient dispersions of microhardness. With decreasing dilution rate, ZrB₂ content and microhardeness increase.     

B4C/Al Composites Processed by Metal-assisted Pressureless Infiltration Technique and its Characterization

In order to improve the wettability between Al melt and B₄C ceramic preform during fabricating B₄C/Al composites by pressureless infiltration technique, trace amount of Ti particulates with high melting point was added into the starting materials as infiltration inducer. A simple and cost-effective method, metal-assisted pressureless infiltration technique, was developed to fabricate light-weight B₄C/Al composites. The microstructure, phases, and mechanical behavior of B₄C/Al composites were characterized by SEM, XRD, and mechanical property test. The density of the as-fabricated B₄C/Al composites was about 2.75 g/cm³ and the relative density of this kind of composites was over 97%. The as-fabricated B₄C/Al composites exhibited rather well wear resistance. The flexural and compressive strengths of the as-fabricated B₄C/Al composites were about 200 MPa and 670 MPa, respectively.     

Preparation and properties of 2D C/ZrB2-SiC ultra high temperature ceramic composites

Two-dimensional C/ZrB₂-SiC composites were fabricated by chemical vapor infiltration (CVI) process combined with slurry paste (SP) method. ZrB₂ was introduced in the matrix by stacking the pasted carbon cloth with ZrB₂-polycarbosilane slurry. After heat-treated at 900 °C, the stacked carbon cloth preform was infiltrated SiC by CVI process to obtain 2D C/ZrB₂-SiC composites. Mechanical properties such as flexural strength and interlaminar shear strength were investigated. The ablation tests were carried out on an oxyacetylene torch flame. The small linear erosion rates indicate that the composites have good ablation resistance properties. These results demonstrate that CVI combined with SP method is a useful way to fabricate 2D C/ZrB₂-SiC composites.     

Synthesis of Zirconium Diboride Films and ZrB<Subscript>2</Subscript>/BC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>N<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Heterostructures

Using mechanochemical synthesis, we have prepared zirconium borohydride, Zr(BH₄)₄, as a precursor for ZrB₂ film growth by chemical vapor deposition. We have carried out the thermodynamic modeling of phase formation processes in the Zr–B–(N)–H and Zr–B–(N)–H–O systems in a wide temperature range, from 100 to 2500°C, at various p_(H2)/p_(Zr(BH4)4) and p_(NH3)/p_(Zr(BH4)4) partial pressure ratios in the starting gas mixtures. A process has been proposed for the growth of zirconium diboride films by Zr(BH₄)₄ decomposition using two techniques: chemical vapor deposition and plasma-enhanced chemical vapor deposition. We also developed a process for the growth of multilayer ZrB₂-and BC _x N _y -based structures.     

Combustion synthesis of ZrB2-SiC composite powders ignited in air

ZrB₂-SiC composite powders have been synthesized by combustion synthesis in air, using a mixture of Zr, B₄C and Si as raw materials. It was found that the air atmosphere has played an important role in the ignition process of the SHS reaction. Three other kinds of ZrB₂-SiC-ZrC composite powders with different ZrC content were also synthesized, and the ignition time has been measured for better understanding the ignition mechanism. As a result, the composite powders with particle size smaller than 1 µm and oxygen content as low as 0.4 wt.% were obtained.     

Synthesis of ultrafine ZrB2 powders by sol-gel process

Ultrafine zirconium diboride (ZrB₂) powders have been synthesized by sol-gel process using zirconium oxychloride (ZrOCl₂·8H₂O), boric acid (H₃BO₃) and phenolic resin as sources of zirconia, boron oxide and carbon, respectively. The effects of the reaction temperature, B/Zr ratio, holding time, and EtOH/H₂O ratio on properties of the synthesized ZrB₂ powders were investigated. It was revealed that ultrafine (average crystallite size between 100 and 400 nm) ZrB₂ powders can be synthesized with the optimum processing parameters as follows: (i) the ratio of B/Zr is 4; (ii) the solvent is pure ethanol; (iii) the condition of carbothermal reduction heat treatment is at 1550°C for 20 min.     

Preparation of B4C-ZrB2-SiC eutectic ceramics by arc melting method

The B₄C-ZrB₂-SiC ternary composites with super hard and high toughness were obtained by arc melting in argon atmosphere. Microstructures were observed by SEM, and phase compositions were analyzed by XRD. The hardness and fracture toughness of ternary composites are 28 GPa and 4.5 MPa·m^1/2. The eutectic mole composition is 0.39B₄C-0.25ZrB₂-0.36SiC, and the eutectic lamellar microstructure is composed of B₄C matrix with the lamellar ZrB₂ and SiC grains.     

Understanding the reaction mechanism of in-situ synthesized (TiC–TiB2)/AZ91 magnesium matrix composites

Magnesium matrix composites reinforced with a network of TiC and TiB₂ compounds have been successfully synthesized via an in-situ reactive infiltration technique. In this process, the ceramic reinforcing phases, TiC and TiB₂, were synthesized in-situ from the starting powders of Ti and B₄C without any addition of a third metal powder such as Al. The molten AZ91 magnesium alloy infiltrates the preform of 3Ti–B₄C by capillary forces. Furthermore, adding different weight percentages of MgH₂ powder to the 3Ti–B₄C preforms was used in an attempt to increase the Mg content in the fabricated composites. The results reveal a relatively uniform distribution of the reinforcing phases in the magnesium matrix with very small amounts of residual Ti, boron carbide and intermediate phases when they are fabricated at 900 °C for 1.5 h using a 3Ti–B₄C preform with 70% relative density. On the other hand, after adding MgH₂ to the 3Ti–B₄C preform, TiC_x and TiB₂ formed completely without any residual intermediate phases with the formation of the ternary compound (Ti₂AlC) at the expense of TiC. The percentage of reinforcing phases can be tailored by controlling the weight percentages of MgH₂ powder added to the 3Ti–B₄C preform. The results of the in-situ reaction mechanism investigation of the Ti–B₄C and Mg–B₄C systems show that the molten magnesium not only infiltrates through the 3Ti–B₄C preform and thus densifies the fabricated composite as a matrix metal, but also acts as an intermediary making the reaction possible at a lower temperature than that required for solid-state reaction between Ti and B₄C and accelerates the reaction rate. The investigation of the in-situ reaction mechanism with or without the addition of MgH₂ powder to the 3Ti–B₄C preforms reveals similar mechanisms. However, the presence of the MgH₂ in the preform accelerates the reaction resulting in a shorter processing time for the same temperatures.     

后热处理对C_f/ZrC复合材料微观结构及性能的影响

以C/C复合材料为基材、Zr_2Cu合金为渗剂,采用低温反应熔渗工艺制备得到碳纤维增强碳化锆陶瓷基复合材料(Cf/ZrC),重点研究后热处理对Cf/ZrC复合材料微观结构及性能的影响。结果表明:经1400~2200℃热处理后,材料密度下降,开孔率增大;材料在后热处理过程中会发生残余富铜熔体的流失、ZrC基体体积分数的增加以及ZrC基体结构的破坏;后热处理造成材料力学性能下降,热处理温度达到2200℃时,材料的弯曲强度保留率仅为52.3%。     

Mechanical properties and microstructure of in situ synthesized ZrB2-ZrN1−x composites

ZrB₂-ZrN_1−x composites were in situ synthesized from Zr and BN powders by hot-pressing at high temperatures. Thermodynamic calculation indicates that ZrN will be formed preferentially than ZrB₂ in Zr-BN system. Three samples with Zr/BN molar ratios of 3:2, 3.5:2 and 2:1 were investigated at temperatures above 1650°C. All mixtures of Zr and BN transformed to ZrB₂-Zr_1−x composites completely without any other detectable phases. Nonstoichiometric zirconium nitride, Zr_1−x, is supposed to be formed in 3.5:2- and 2:1-samples. The microstructural morphology of well-sintered ZrB₂-Zr_1−x composites is characterized by quadrate column-shaped ZrB₂ distributed evenly in the interwoven acicular Zr_1−x matrix. A certain amount of hollow rectangular-shaped ZrB₂ with open ends is found in 3.5:2-sample hot-pressed at 1700°C, while some large spherical particles with lots of acicular Zr_1−x sticked on its surface are observed in 2:1-sample hot-pressed at 1800°C. Excessive Zr compared to the stoichiometric Zr/BN molar ratio of 3:2 will facilitate the densification process. Acicular Zr_1−x is apparently beneficial to the improvement of bending strength and fracture toughness of ZrB₂-Zr_1−x composites.     

Combination of SHS and SPS Techniques for fabrication of fully dense ZrB2-ZrC-SiC composites

A novel method for the fabrication of fully dense ZrB₂-ZrC-SiC Ultra High Temperature Ceramic (UHTC) materials is proposed. It consists of first synthesizing ZrB₂-40 vol.% ZrC-12 vol.% SiC powders by Self-propagating High-temperature Synthesis (SHS) and subsequently consolidating them by Spark Plasma Sintering (SPS). Specifically, when starting from Zr, B₄C, Si, and graphite, the SHS technique leads to the complete conversion of reactants to the desired product. In addition, the use of the SPS apparatus allows for the full consolidation of the SHS powders. This result is achieved under the optimal conditions of 10 min total time and with a maximum temperature of 1800 °C. The proposed method is particularly rapid and convenient as compared to other techniques available for the preparation of analogous materials and for the consolidation of commercial ZrB₂, ZrC, and SiC, using the same SPS conditions.     

Near net-shape, ultra-high melting, recession-resistant ZrC/W-based rocket nozzle liners via the displacive compensation of porosity (DCP) method

Dense, near net-shaped ZrC/W-based composites have been fabricated at modest temperatures and at ambient pressure by a reactive infiltration process known as the Displacive Compensation of Porosity (DCP) method. Porous WC preforms with hourglass shapes (for rocket nozzle liners) were produced by gel casting, whereas simple bar-shaped preforms were produced by uniaxial pressing. The porous preforms were exposed to molten Zr₂Cu at 1200–1300°C and ambient pressure. The Zr₂Cu liquid rapidly infiltrated into the preforms and underwent a displacement reaction with the WC to yield a more voluminous mixture of solid products, ZrC and W. This displacement reaction-induced increase in internal solid volume filled the prior pore spaces of the preforms (displacive compensation of porosity) to yield dense, ZrC/W-based composites. Because the preforms remained rigid during reactive infiltration, the final composites retained the external shapes and dimensions of the starting preforms. A DCP-derived, ZrC/W-based nozzle insert was found to be resistant to the severe thermal shock and erosive conditions of a Pi-K rocket motor test. The DCP process enables dense, ceramic/refractory metal composites to be fabricated in complex and near net shapes without the need for high-temperature or high-pressure densification or for extensive machining (i.e., relatively expensive processing steps are avoided).     

Spark plasma sintering of UHTC powders obtained by self-propagating high-temperature synthesis

Fully dense ZrB₂–SiC and HfB₂–SiC ultra-high-temperature ceramics (UHTCs) composites are fabricated by first synthesizing via self-propagating high-temperature synthesis (SHS) the composite powders from B₄C, Si, and Zr or Hf reactants, and subsequently consolidating the product by spark plasma sintering (SPS) without the addition of any sintering aid. It was found that the SHS technique leads to the complete conversion of reactants to the desired products and the SPS allows for the full consolidation (>99.5% relative density) under the optimal operating conditions of 1800 °C/20 min/20 MPa and 1800 °C/30 min/20 MPa, for the cases of ZrB₂–SiC and HfB₂–SiC, respectively. Based on the results reported in this work, it can be stated that the combination of SHS and SPS methods represents a particularly rapid and convenient preparation route (lower sintering temperature and processing time) for UHTCs as compared to the techniques available in the literature for the fabrication of analogous products.