•A fine modified microstructure of high chromium white irons was achieved by in situ formed TiCx.•The formed TiCx from the decomposition of Ti3AlC2 is round, about 2 μm in size.•The small TiCx grains pinned at the M7C3 carbide growth front and refined the carbide structure.•The influence factors on the microstructural development and the dispersion of TiCx have been investigated.High chromium white irons have excellent abrasion resistance due to the presence of hard eutectic M7C3 (M = Fe, Cr) carbides in a ferrous matrix. However, the appearance of large and coarse M7C3 carbides is detrimental to the performance of high chromium white irons. To modify the microstructure, in situ decomposition of Ti3AlC2 to TiCx which is expected to inhibit the growth of M7C3 in a high chromium white iron has been adopted in the present study. The in situ formed TiCx effectively refined the M7C3 carbide structure. The influences of Ti3AlC2 content, sintering temperature and heating rate on the microstructural evolution of high chromium white iron were investigated. Influencing factors on the size and dispersion of the TiCx grains in the ferrous matrix have been analyzed. A modified microstructure with a fine eutectic M7C3 structure and a homogenous distribution of small round TiCx can be achieved by optimization of temperature and dwell time. X-ray diffraction, differential scanning calorimetry and scanning electron microscopy were used to investigate phase composition, phase transition and microstructure of materials.Download high-res image (617KB)Download full-size image

•Influence of thermal shock on electrical conductivity of Ti2SnC was investigated.•Abnormal recovery of electrical conductivity occurred after quenching at 600 °C;•Filling of small cracks by Sn is the main mechanism for electrical conductivity recovery.Some ternary carbide and nitride ceramics have been demonstrated to exhibit abnormal thermal shock behavior in mechanical properties. However, the influence of thermal shock on other properties is not clear. This work reports on the influence of thermal shock on electrical conductivity of Ti2SnC as a representative member of ternary carbides. Abnormal change in electrical conductivity was first demonstrated during quenching Ti2SnC in water at 500–800 °C. The residual electrical conductivity of the quenched Ti2SnC gradually decreased with increasing temperature, but abnormally increased after quenching at 600 °C. The microstructure of surface cracks was characterized. The main mechanism for the abnormal electrical conductivity recovery is that some narrow branching cracks are filled by metallic Sn precipitating from Ti2SnC.Download high-res image (304KB)Download full-size image

Self-healing ceramics are able to heal cracks through an oxidative healing mechanism at high temperature in oxidizing environments with the recovery of original performance and functionality. However, the oxidation induced repair may be impossible when the ceramics are used in vacuum or inert atmospheres with low oxygen partial pressures. So far little work has been done on crack healing in vacuum or inert atmospheres. Here we report on the crack healing of a Ti2SnC ceramic in vacuum by a precipitation induced repair mechanism. Cracks induced by thermal shock in Ti2SnC are completely filled by only metallic Sn at temperatures above 800 °C for only 1 h in vacuum. The electrical conductivity of healed materials is fully recovered, and it even exceeds the initial conductivity. This work might bring a new wave of research on crack healing behavior of ceramics in low oxygen partial pressure environments.

Ceramics are able to heal cracks and flaws by triggering local sintering (internal cracks) or oxidation reaction (surface cracks) which may induce the recovery of mechanical strength. Crack healing in ceramics, however, is limited to micro-sized cracks and requires elevated temperatures (above 1000 °C) and long time periods even up to 100 h. In this work we report on accelerated crack healing of a Ti2SnC ceramic which is capable to repair thermal shock induced cracks at a relatively low temperature of 800 °C within only 1 h. After healing treatment both the low flexural strength and electrical conductivity measured on the damaged material after quenching were almost fully recovered. Furthermore, Ti2SnC exhibits repeatable healing capability which offers a high potential for developing durable ceramics with prolonged lifetime under harsh thermal conditions.

This work reports on the fabrication and high temperature ablation property of a new ZrC/Cr2AlC composite. The ZrC/Cr2AlC composite was obtained by hot pressing a mixture of 15 vol% ZrC and 85 vol% Cr2AlC powders at 1300 °C with 20 MPa for 1 h in Ar atmosphere. The composite had a flexural strength of 622 MPa, higher than 400 MPa for Cr2AlC. The high temperature ablation behavior of the composite was investigated using the oxyacetylene torch ablation test. During oxyacetylene torch testing, the composite underwent a series of thermal decomposition and oxidation. Microstructure and composition of the synthesized composite before and after the ablation test were characterized with scanning electron microscopy and X-ray diffractometry techniques.

•Thermal shock behavior of Cr2AlC in water, oil and molten salt was investigated.•Abnormal thermal shock behavior was observed during quenching in different media.•Crack healing is still the main mechanism for the unique thermal shock resistance.•The unique property makes Cr2AlC promising for high-temperature applications.Many ternary ceramics exhibit an abnormal thermal shock behavior as quenched in water. It is not clear that the unique thermal shock behavior remains valid during quenching in other quenching media. This work reports on the investigation of the thermal shock behavior of Cr2AlC as a representative member of the ternary ceramics in various quenching media. The quenching tests revealed that the abnormal thermal shock behavior of Cr2AlC was observed during quenching in water, oil and molten salt. Crack healing is the main mechanism responsible for the unusual thermal shock behavior.

•A Ti2AlC soft ceramic has been surface-treated by plasma nitriding.•A continuous surface coating mainly composed of TiN was formed.•The surface coating is harder than the matrix.•Scratch test demonstrates a strong adhesion of the coating to the substrate.Plasma nitriding treatment was used to improve the surface hardness of a Ti2AlC soft ceramic due to its relatively low hardness of ~ 3 GPa. A continuous coating was formed by plasma nitriding Ti2AlC at 800°C for 4 h in a mixture of 75 vol.% N2 and 25 vol.% H2 gas atmosphere. The surface layer is about 6 μm in thickness and composed of TiN with a small amount of AlN. The hardness and the adhesion of the coating to the substrate were evaluated with Vickers hardness and scratch tests. The results showed that a strong coherent coating was formed and surface hardness was improved. The plasma nitriding treatment is a promising technique to improve the surface properties of Ti2AlC at relatively low temperatures.

This work reports the cyclic thermal shock behaviour of a Cr2AlC ceramic. The cyclic thermal shock tests were performed by quenching from 825 to 1325 °C into ambient temperature water. The material withstood five thermal shock cycles without catastrophic thermal failure. The three-point bending test showed that its residual strength after five cycles of thermal shock decreased from 400 MPa of the initial strength to about 68 MPa as the quenching temperature rose to 1200 °C, and then increased to ~80 MPa after quenching from 1300 °C. The microstructure of the quenched samples was characterised with scanning electron microscopy and optical microscopy. The main mechanism for the enhanced thermal shock resistance of Cr2AlC has been analysed.

A highly attractive self-healing material would be one which combines excellent mechanical properties with a multiple healing capability. Self-healing ceramics have been studied for over 40 years to obtain some performance recovery and to prevent material failure during service, but so far only materials with the capability of single healing event per damage site have been realized. Here we report on a self-healing Ti2AlC ceramic capable of repeatedly repairing damage events. The Ti2AlC ceramic achieves at least seven healing cycles after repeated cracking at a given location. The main healing mechanism at high temperature is the filling of the cracks by the formation well adhering α-Al2O3 and the presence of some rutile TiO2. For healed samples, the flexural strength returned or even slightly exceeded the virginal strength. The fracture toughness recovery has been quantified for multiple healing cycles.

A dense SiC/Ti3Si(Al)C2 composite was synthesized by in situ hot pressing powders of Si, TiC and Al as a sintering additive at 1500 °C for 2 h under 30 MPa in Ar atmosphere. This composite has a fine-grained and homogeneous microstructure with grain sizes of 5 μm for Ti3Si(Al)C2 and of 1 μm for SiC. The SiC/Ti3Si(Al)C2 composite possesses an improved oxidation resistance, with parabolic rate constants of 4.57 × 10−8 kg2/m4/s at 1200 °C and 1.31 × 10−7 kg2/m4/s at 1300 °C. This study provides an experimental evidence to confirm the formation of amorphous phases in the oxide scale of the SiC/Ti3Si(Al)C2 composite. Microstructure and phase composition of the SiC/Ti3Si(Al)C2 composite and oxide scales were identified by X-ray diffractometry and scanning electron microscopy. The mechanism for the enhanced oxidation resistance has been discussed.

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr2AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot pressing at 30 MPa and different temperatures for 1 h in Ar atmosphere. The results show that a dense Cr2AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr2AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr2AlC (grain size of about 35 μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr2AlC were determined and compared with the values for the coarse-grained Cr2AlC.

A new reaction route with AlCr2 and C as starting materials has been developed to produce Cr2AlC. A Cr2AlC bulk ceramic was achieved by hot pressing the AlCr2 and C powders at 1400 °C with 20 MPa for 1 h in Ar. The mechanism to form Cr2AlC in a temperature range of 1050–1400 °C was studied. It was confirmed that Cr2AlC is formed directly by a reaction between C and AlCr2. The reaction process, phase composition and microstructure were characterized with differential thermal analysis, X-ray diffractometry and scanning electron microscopy. The produced Cr2AlC ceramic is stable up to 1500 °C in an Ar atmosphere, but decomposes into Al8Cr5 and Cr23C6 above 1500 °C.