Study on the plasma erosion resistance of ZrO2p(3Y)/BN–SiO2 composite ceramics

ZrO_2p(3Y)/BN–SiO₂ composite ceramics were prepared by hot pressed sintering. The Kr plasma was incident normally to the sample surface to measure the plasma erosion resistance, and XRD, TEM, SEM were used to characterize the phase composition and morphology of composite ceramics before and after Kr plasma erosion. The composites were composed of h-BN, amorphous SiO₂, m-ZrO₂ and t-ZrO₂, which keep stability in the process of Kr plasma sputtering. Two kinds of zirconia phases (t-ZrO₂, m-ZrO₂) and lamellar h-BN evenly distributed in ZrO_2p(3Y)/BN–SiO₂ composite ceramics, and the lath microstructures were observed in m-ZrO₂ particles. The plasma erosion resistances of ceramic composites increase significantly with the increase of ZrO₂ content. Pits with diameter sizes of several micrometers were observed on the surface of composites after plasma sputtering, which indicate that one or several grains are sputtered out during the plasma sputtering process. The edges of the grains become smoother for the preferential sputtering to the sharp edge of the grains during plasma sputtering.     

Properties of B4C/Al–B4C composite with a two-layer structure

In order to obtain a material with a promising bulletproof performance, a two-layer structure composite consisting of B₄C/Al-B₄C was obtained using a two-step method for both hot pressing and infiltration aluminum in vacuum. Before aluminum infiltration the B₄C porous layer of the two-layer preform looked like a three-dimensional network of interconnected capillaries. For the B₄C ceramics layer the microstructure showed no apparent change before and/or after aluminum infiltration. The two-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C-Al material.     

Microstructure and properties of ZrO2-, TiO2-doped Ca–Si–B based ceramics

The microstructure, sintering and dielectric properties of ZrO₂-, TiO₂-doped Ca–Si–B based ceramics prepared by solid-phase process were investigated, and the effects of ZrO₂, TiO₂ content on these performances were analyzed. The Ca–Si–B based ceramics without additive (ZrO₂ or TiO₂) showed a high sintering temperature (1,100?°C) and had the dielectric properties: dielectric constant (ε_r) of 8.38, dielectric loss (tanδ) of 1.51?×?10^?3 at 1?MHz, and volume density of 2.47?g/cm³. The addition of ZrO_2, TiO₂ was revealed to lower the sintering temperature of Ca–Si–B based ceramics to 1,000?°C and enhance the sintering and dielectric properties: ρ?=?2.61?g/cm³, ε_r?=?5.85, tanδ?=?1.59?×?10^?4 (1?MHz) with ZrO₂ addition, and ρ?=?2.65?g/cm³, ε_r?=?6.12, tanδ?=?6.4?×?10^?4 (1?MHz) with TiO₂ addition, which are superior to the pure Ca–Si–B. The results show that ZrO₂, TiO₂ as nucleating agents, are conducive to the precipitation of crystals, thus decrease the sintering temperature and improve the dielectric properties of Ca–Si–B based ceramics.     

Microstructure and mechanical properties of a three-layer B4C/Al–B4C/TiB2–B4C composite

A three-layer structure material, consisting of B₄C/Al, B₄C/TiB₂ and B₄C composites, was obtained using a two-step method for both hot pressing and aluminum infiltration in vacuum. The three-layer B₄C/Al–B₄C/TiB₂–B₄C composite showed good interfacial bonding. Before aluminum infiltration the B₄C porous layer in the three-layer preform looked like a three-dimensional network of interconnected capillaries. The microstructures of both B₄C/TiB₂ and B₄C layers showed no apparent changes before and/or after aluminum infiltration. The three-layer composite showed improved fracture toughness than that of B₄C material and higher comprehensive hardness than that of B₄C/Al material.     

Effects of nucleating agents on crystallization and microstructure of fluorophlogopite mica-containing glass–ceramics

The alteration of crystallization behavior, microstructure, and thermal properties of fluorophlogopite mica-containing glass–ceramics by nucleating agent is systematically studied. TiO₂, TiO₂ + ZrO₂, and ZrO₂ have been doped as the nucleating agents in the SiO₂–MgO–Al₂O₃–B₂O₃–K₂O–MgF₂ (BMAPS) glass system and prepared by the melt-quench technique. The glass without nucleating agent is also prepared to ascertain the influence of nucleating agent. Addition of nucleating agents effectively increases the softening as well as glass transition temperatures. From the DSC study, it is found that the fluorophlogopite mica crystallization exotherm exhibited in the temperature range 800–850 °C and the activation energy varies in the range 167–182 kJ/mol. The opaque mica glass–ceramics are derived from these BMAPS glasses by a controlled heat treatment process and heat treatment at 1050 °C is found to be optimum. The mica crystals were identified as fluorophlogopite for all the four BMAPS glasses by X-ray powder diffraction (XRD) and subsequently confirmed by FTIR spectroscopy. Excellent matching with fluorophlogopite crystal was obtained in Zirconia-containing glass–ceramic as perceived from the XRD and FTIR studies. The microstructure of interlocked card-like mica flake crystals is found to form as seen from scanning electron microscopy, and such microstructure is obtained when ZrO₂ has been used as nucleating agent. Glass–ceramic without nucleating agent possesses Vickers hardness value 4.58 Gpa and it is increased with addition of the nucleating agent (5.67–6.56 GPa), ZrO₂-containing glass–ceramic possess lower hardness (5.67 GPa) and better machinability. Therefore, ZrO₂ is the most efficient nucleating agent to generate fluorophlogopite mica in these glass–ceramics useable for SOFC applications.     

Electroless deposition of Ni–W–P–B4C nanocomposite coating on AZ91D magnesium alloy and investigation on its properties

In the present work, electroless deposition of quaternary Ni–W–P–B₄C composite coatings on AZ91D magnesium alloy was investigated. The coatings were characterized to study their microstructure, crystallite size, morphology, microhardness and corrosion resistance and compared with Ni–P and Ni–P–B₄C composite coatings, prepared with the same method. The hardness of the Ni–W–P–B₄C composite coatings was around 1290 MPa which was more than that of the Ni–P and Ni–P–B₄C coatings (about 700 and 1200 MPa, respectively). According to polarization test results, the Ni–W–P–B₄C composite coating exhibits less and more corrosion rates with respect to the Ni–P–B₄C and the Ni–P coatings, respectively. X-ray diffraction (XRD) analysis results for the Ni–W–P–B₄C coating showed that the Ni–W–P–B₄C coating has a combination of amorphous and nanocrystalline structures. Also, Williamson–Hall analysis on the X-ray patterns revealed that the Ni–W–P–B₄C coating has an average crystallite size of 1.5 nm.     

Low-Temperature Synthesis of Boron Carbide Ceramics

This study has been the first to demonstrate the possibility of producing boron carbide ceramics from coarse (D = 25–150 μm) B₄C powder (which is impossible to sinter by conventional methods) through infiltration with molten silicon and subsequent treatment within the field of the controlled temperature gradient. This produces yields a composite ceramics B₄C–SiC–Si with a hardness of 26 to 35 GPa and a splitting tensile strength of 110 to 170 MPa. The influence of the velocity of movement of the temperature gradient on the structure, phase composition, and properties of the prepared composites has been studied.     

Preparation of graded zirconia–CaP composite and studies of its effects on rat osteoblast cells in vitro

Hydroxyapatite (HAP) has excellent biocompatibility and bone bonding ability, but it is mechanically weak and brittle. To overcome this problem, we prepared a graded composite with calcium phosphide (CaP, decomposed from HAP during sintering) coating on the surface of zirconia (ZrO₂) ceramics. The mechanical properties and microstructure characteristics were studied with various techniques. The biocompatibility of graded ZrO₂–CaP composite was examined with rat osteoblast cells (OB cells) in vitro. Its effects on the production of alkaline phosphatase (ALP), Interleukin-6 (IL-6) and Growth-transforming Factor-β (TGF-β) by the OB cells were measured. The results showed that the mean tensile strength of the graded ZrO₂–CaP composites was 17.8 MPa, the maximum bending strength was 1112.24 MPa, and K_IC was 7.3–11.4 MPa·m^1/2, indicating that the composite was physically strong for use as an implant material. The ALP activity, IL-6 and TGF-β concentrations of the graded composite treated OB cells were much higher than that of the pure ZrO₂ treated group. There was no significant difference in ALP activity, the IL-6 and TGF-β concentrations between the graded ZrO₂–CaP composite group and HAP. The cytotoxicity of the composite material to rat fibroblast cells was insignificant. The graded zirconia–CaP composite greatly facilitated the proliferation and differentiation of rat OB cells in vitro, demonstrating excellent biocompatibility.     

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.     

Design and fabrication of porous ZrO2/(ZrO2 + Ni) sandwich ceramics with low thermal conductivity and high strength

3Y–ZrO₂/(3Y–ZrO₂ + Ni) sandwich ceramics were fabricated through cold isostatic pressing and pressureless sintering. Porous 3Y–ZrO₂ ceramics with large connecting open pores and permeability were used as interlayers for insulation, whereas outer metal–ceramic layers were used as bearing loads. Microstructures and properties of the porous ZrO₂ and ZrO₂/(ZrO₂ + Ni) sandwich ceramics were investigated in detail. The ZrO₂/(ZrO₂ + Ni) sandwich ceramics exhibited better mechanical properties than the monolithic porous ZrO₂ ceramics at the same low thermal conductivity (approximately 0.85 W/m K). The mechanical properties of the sandwich ceramics were influenced by metal toughening and sintering-induced residual thermal stress.     

Mechanical properties of zirconia-alumina composite ceramics prepared from Zr-Al metallo-organic compounds

The mixture of a Zr-Al metallo-organic compound and Al₂O₃ powder yields dense ZrO₂-Al₂O₃ composite ceramics. The fraction of the tetragonal ZrO₂ phase in as-sintered ZrO₂-Al₂O₃ ceramics is almost 100% and the ZrO₂ grains at about 500 nm in diameter are dispersed in the matrix. The ceramics have high fracture toughness and bending strength.     

Strategical parametric investigation on manufacturing of Al–Mg–Zn–Cu alloy surface composites using FSP

Friction stir processing (FSP) is a unique approach being presently researched for composite fabrication. In the present investigation, Al-B₄C surface composite was fabricated through FSP by incorporating B₄C powder particles into Al–Mg–Zn–Cu alloy (AA 7075) matrix. The influence of varying powder particle reinforcement strategies on the microstructure, powder distribution, microhardness, and wear resistance of the surface composite is reported. In addition, AA 6061/B₄C composites were prepared using the same parameter set and the powder distribution in the composite was compared to that in the AA 7075/B₄C composite. More homogeneous dispersion of B₄C powder was observed in AA 6061 as compared to AA 7075 substrate. Among the prepared AA 7075/B₄C composites, the best B₄C powder distribution was detected in samples processed using fine powder and incorporating the change in stirring direction between passes. The hardness and wear resistance of the prepared composites were almost doubled attributing to several strengthening mechanisms and B₄C powder distribution in the AA 7075 matrix.     

Microstructure and mechanical properties of hot pressed Al2O3–ZrO2 ceramics prepared from ultrafine powders

Abstract

The microstructure and mechanical properties of hot pressed Al₂O₃–ZrO₂ ceramics prepared from ultrafine powders were studied by means of hardness and fracture toughness tests, X-ray diffraction, scanning and transmission electron microscopy, and electron probe microanalysis. Experimental results show that Al₂O₃–ZrO₂ ceramics combine the high modulus of Al₂O₃ and phase transformation toughening of ZrO₂ and give good mechanical properties. The fracture toughness of the samples increases monotonically with increasing ZrO₂ content. When the content of ZrO₂ is low, the ZrO₂ particles are surrounded by Al₂O₃ grains and the matrix is thus strengthened, but when the content of ZrO₂ is high, the microcracks resulting from the tetragonal monoclinic phase transformation decrease the strength of the material. The relative density of the samples also increases with increasing ZrO₂ content, which is beneficial to both strength and toughness. Banded abnormal growth of the Al₂O₃ grains parallel to the hot pressing plane was found in the samples. This phenomenon is considered to be a result of the priority of Al₂O₃ grain growth and aggregation of ZrO₂ along the pressing direction in the later stages of hot pressing.

MST/1359     

Preparation of zirconium pyrophosphate bonded silicon nitride porous ceramics

Abstract

A new method for preparing high bending strength porous silicon nitride ceramics with controlled porosity was developed using a pressureless sintering technique, using zirconium pyrophosphate as a binder. The fabrication process was described in detail and the sintering mechanism of porous ceramics was analysed by an X-ray diffraction method. The microstructure and mechanical properties of the porous Si₃N₄ ceramics were investigated, as a function of the content of ZrP₂O₇. The resultant porous silicon nitride ceramics sintered at low temperature (1000 and 1100°C) showed fine micropore structure and a high bending strength. Porous silicon nitride ceramics with porosity of 34–47%, a bending strength of 40–114 MPa and a Young's modulus of 20–50 GPa were obtained.     

Formation and sintering of 75 mol% alumina/25 mol% zirconia (2–3.5 mol% yttria) composite powder prepared by the hydrazine method

Al₂O₃/Y₂O₃-doped ZrO₂ composite powders with 25 mol% ZrO₂ have been prepared by the hydrazine method. As-prepared powders are the mixtures of AlO (OH) gel solid solutions and amorphous ZrO₂. The formation process leading to -Al₂O₃/t-ZrO₂ composite powders is investigated. Hot isostatic pressing has been performed for 1 h at 1500 °C under 196 MPa. Dense ZrO₂-toughened Al₂O₃ (ZTA) ceramics with homogeneous-dispersed ZrO₂ particles show excellent mechanical properties. The toughening mechanism is discussed.     

Structural stability and non-catalytic nucleation inhibition effect of Si–Zr–B mould coating on undercooled superalloy melt

Abstract

The investment moulding technique was first adopted to prepare a SiO₂–ZrO₂–B₂O₃ (Si–Zr–B) substrate layer on the inner surface of the mould, by employing SiO₂ glass dust and ZrO₂ powder, SiO₂–ZrO₂ sol, and analytical grade H₃BO₃ as refractory material, binder, and softening agent, respectively. Then using sol–gel processing, seven layers of Si–Zr–B film of the same formula as the aforementioned Si–Zr–B substrate layer were compounded with the substrate layer step by step. After glassing treatment at 850°C for 60 min, this film transformed into a glass lined coating. It was shown from X-ray diffraction analysis that, after holding it at a temperature of 1500°C for 30 min, the amount of crystallinity in the Si–Zr–B coating was about 1–3% (vol.-%). Finally, the undercooling experiment showed that a large undercooling (up to 140 K) was achieved in a DD3 (Ni–Cr–Mo–Al–Ti–Co–W) single crystal superalloy melt in this coated mould. So it is concluded that a Si–Zr–B coating has got a good structural stability at high temperature and provides ideal non-catalytic nucleation inhibition for an undercooled superalloy.     

Reactive sintering of B_4C-TaB_2 ceramics via carbide boronizing:Reaction process,microstructure and mechanical properties

Carbide boronizing is a promising approach to obtain fine grained boron carbide based ceramics with improved mechanical properties. In this work, reaction process, microstructural characteristics and mechanical properties of BxC-TaB_2(x = 3.7, 4.9, 7.1) ceramics were comprehensively investigated via this method. Dense BxC-TaB_2 ceramics with refined microstructure were obtained from submicro tantalum carbide and boron powder mixtures at 1800?C/50 MPa/5 min by spark plasma sintering. The stoichiometry of boron carbide was determined from lattice parameters and Raman shift. It was found that uniformly distributed TaB_2 grains in the BxC matrix is favor of the densification process and restricting grain growth.Besides, planar defects with high density were observed from the as-formed B7.1 C grains and transient stress was considered to contribute to the densification involved with plastic deformation. Microstructural observations indicate the dissolution of oxygen in the TaB_2 lattice and most of the B7.1 C/TaB_2 phase boundaries were clean. Owing to the highly faulted structure and finer grain size, as-obtained BxC-TaB_2 ceramics exhibit high Vickers hardness(33.3–34.4 GPa at 9.8 N) and relatively high flexural strength ranging from 440 to 502 MPa.     

Influence of zirconium addition on final properties of K0.5Na0.5NbO3-based ceramics

Microstructure, electrical and dielectric properties of K_1/2Na_1/2NbO₃ (KNN) modified with ZrO₂ were investigated. Powders were obtained by a conventional solid-state method. Samples doped with 0–2 mol% of ZrO₂ were sintered at 1,125 °C for 2 h. Through XRD spectra, the perovskite structure was observed, in addition to small peaks corresponding to secondary phases. It was also determined that zirconium drastically changed the microstructure and grain size of KNN ceramics. The addition of upto 1.0 mol% of Zr⁴⁺ produced a softening effect in the ferroelectric properties of the material, and increased its density. Conversely, samples prepared with contents higher than 1.0 mol% reduced piezoelectric and dielectric properties.     

Effect of ZrB2 and Polyvinyl Butyral Content on the Oxidation Resistance of ZrB2–SiC Coatings Produced by Slurry Brushing Method

ZrB₂–SiC coatings are prepared on the surface of graphite by slurry brushing method to improve the oxidation resistance. Effects of ZrB₂ content and polyvinyl butyral (PVB)–ethanol solution concentration on microstructure and static oxidation behavior of the ZrB₂–SiC coatings are investigated at 1200 °C in air. The results indicate that increasing ZrB₂ content improves the oxidation resistance of the coatings. When ZrB₂ content increases from 30 to 45 wt%, weight loss rates of the coated samples after oxidation at 1200 °C for 120 min decrease from −0.92% to −1.67%. Increasing binder solution concentration raises component content in the coatings. As PVB–ethanol binder concentration increases from 0.025 to 0.075 g mL⁻¹, weight loss rates of the coated samples after oxidation at 1200 °C for 120 min decrease from 0.32% to −0.38%. Excellent oxidation resistance of ZrB₂–SiC coating is attributed to self-sealing ability of B₂O₃ and borosilicate glass. The composite glass can inhibit oxygen diffusion by filling defects in the coating promptly. The borosilicate glass phase can enhance the fluidity of the composite glass. ZrO₂ and ZrSiO₄ particles restrict the growth of the microcrack, which improves the oxidation resistance of ZrB₂–SiC coating.     

Densification behaviour and mechanical properties of pressureless-sintered B4C–CrB2 ceramics

B₄C based ceramics composites with 0–25 mol% CrB₂ were fabricated by pressureless sintering in the temperature range 1850°C to 2030°C. The CrB₂ addition enhanced the densification of B₄C due to the CrB₂–B₄C eutectic liquid phase formation. Both a high strength of 525 MPa and a modest fracture toughness of 3.7 MPa m^1/2 were obtained for the B₄C–20 mol% CrB₂ composite with a high-relative density of 98.1% after sintering at 2030°C. The improvement in fracture toughness is thought to result from the formation of microcracks and the deflection of propagating cracks resulting from the thermal expansion mismatch of CrB₂ and B₄C.