The Influence of Ni Addition on the Microstructures and Mechanical Properties of Al2O3–TiN–TiC Ceramic Materials
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Al2O3–TiN–TiC ceramic materials with different Ni content were fabricated by hot-pressing technique. Mechanical properties such as flexural strength, Vickers hardness and fracture toughness were obtained by three-point-bending test and Vickers indentation method. The microstructures of the sintered body were observed by scanning electron microscopy. The effects of Ni content on the microstructures and the mechanical properties of Al2O3–TiN–TiC ceramic materials were investigated. The results indicated that the addition of Ni could change the topography of the microstructures which displayed various core–shell structures. Moreover, the fracture mode changed with the increment of Ni content. When the addition of Ni was 3 vol%, the flexural strength and fracture toughness were the greatest and the improvement could be explained by the unique microstructures of sintered bodies.
KeywordsNi addition Mechanical properties Microstructure Al2O3–TiN–TiC ceramic materials
Alumina (Al2O3) ceramics are the most produced and widely applied ceramic materials due to the excellent high-temperature behavior, good chemical stability, low price and so on [1, 2]. Although Al2O3 ceramics are important structural materials, the inherent brittleness has limited their applications. By introducing some additives, the aluminum composites have better mechanical properties especially better fracture toughness than pure alumina ceramics. The common additives include reinforcements, sintering aids and binders.
The addition of strong reinforcements can substantially increase the fracture toughness of alumina ceramics. Both the domestic and foreign researches indicate [3, 4, 5, 6, 7, 8, 9] that the addition of TiC and TiN particles into Al2O3 matrix can improve the flexural strength and the fracture toughness. Titanium carbide (TiC) possesses high hardness (32 GPa) and high melting point (3140 °C). TiC particles can stop the original crack from growing as a result of pinning effects [3, 4]; therefore, the strength, hardness, thermal shock stabilities and thermal conductivity of alumina ceramics can be increased. The toughening mechanism is revealed by the crack deflection and bridging observed in the microstructure of Al2O3–TiC composites . As for TiN particles reinforced alumina composites [7, 8, 9], it is found that Al2O3–TiN ceramic materials have better thermal shock stabilities and better toughness than Al2O3–TiC ceramic materials. Zbiginiew and Jerzy  investigated the microstructure, mechanical properties, oxidation resistance, wear resistance, and electrical conductivity of Al2O3–TiN particulate ceramic composites; the results indicated that the introduction of TiN particles could improve not only the electrical conductivity of monolithic alumina but also the mechanical properties and it can be applied as wear-resistant materials. The toughening mechanisms by TiN are mainly related to the cracks; for example, the crack deflections and crack pinning mechanism were revealed by Li et al. , the crack tilting and twisting mechanism was revealed by Shen et al. , and the crack propagating along the grain boundary was observed by Qiao et al. . Recent work showed that when adding both TiN and TiC particles into alumina ceramics, the composite had excellent hardness and fracture [11, 12, 13], and the cutting performance was better than LT55.
MgO is the most common sintering additive of Al2O3-based ceramic materials. There are several mechanisms about the role of MgO in the sintering of alumina [14, 15, 16, 17, 18, 19]. It is proven that MgO addition can significantly control the evolution of microstructure during sintering process. MgO can reduce the grain boundary mobility by a solute drag mechanism [14, 15], and as a result, it can enhance the densification rate. Meanwhile, MgO can reduce anisotropies in the surface and suppress the abnormal grain growth so as to decrease effects of inhomogeneous densification . Previous studies [20, 21] have proven that the addition of MgO can change the phase composition of Al2O3–TiN–TiC sintered ceramic materials which displays with diverse solid solutions and intermetallic compounds, and MgO addition can also change the microstructure morphology by making the crack path complex.
By incorporating metal binding phase into the ceramic matrix, both fracture toughness and flexural strength can be improved . Molybdenum [23, 24, 25, 26, 27, 28, 29, 30, 31, 32] and nickel [33, 34, 35, 36], are commonly used for Al2O3-matrix ceramics.
Mo has good physical and mechanical comparability with Al2O3 [23, 24]. Sivakumar  explained that the increase in flexural strength was due to smaller initial flaws in mullite/Mo composites for lower Mo contents and due to plastic deformation of Mo phase for higher Mo contents. On the other hand, the toughening mechanism was regarded as the frontal process zone toughening and crack bridging. Partial debonding in the mullite–Mo interface, giving rise to plastic deformation of Mo phase, also enhanced the toughness. Konopka et al.  found that the fracture toughness of ceramic materials was excellent as the result of the interactions between the Mo particles and the crack, such as surrounding of a particle by the crack and passing through a particle. Another important beneficial effect of Mo is due to improving the wettability of the Ni binder on the carbide phase [27, 28, 29, 30]. Improved wettability results in a decrease in detrimental microstructural defects and an increase in the interphase bond strength and phase uniformity. Rim structure is easy to form with Mo addition, and the structure can prevent the aggregation and growth of hard phase particles. The previous study of the Al2O3–TiN–TiC ceramic tool materials showed that the Mo additive could make the shape of hard phase become blunt, and the intergranular fracture became the major fracture with the increment of Mo content, which was beneficial to the fracture toughness .
The ductility of Ni is outstanding, when the size of Ni particles in the microstructure is large, and the plastic deformation caused by ductile nickel particles can improve the toughness of alumina matrix composites. Meanwhile, the weak bonding at the Al2O3/Ni interface can promote partial debonding and contribute further to toughening. When the size of nickel particles is very small, minimum amount of Ni addition can refine the Al2O3 matrix grains and strengthen the grain boundary. Thus, the strength of Al2O3 matrix is consequently enhanced . The melting point of Ni is 1453 °C, and the suitable addition of Ni can produce liquid phase in the sintering process and induce the anisotropic grain growth, which can improve the fracture toughness of Al2O3 ceramic materials . Fahrenholtz et al.  presented that the ductile deformation of Ni can promote energy absorption of crack, and crack bridging mechanism also enhances the toughness. Lu et al.  also proved that Ni could improve fracture toughness and the flexure strength of alumina. The study indicated that the strengthening of composites was attributed to the refinement of microstructure, and the toughening of composites was attributed to the void-induced crack tip blunting effect. In Tuan’s research , the strength of Al2O3 is enhanced significantly by adding nano-sized Ni particles. For example, the strength of alumina has been enhanced by 54% after the addition of 5 vol. % of 100 nm Ni particles.
For a long time, the investigations of the Ni addition in the ceramic materials have mainly focused on the Al2O3–Ni composites [33, 34, 35, 36], but there were few reports about complex material system. On the other hand, the Al2O3–TiN–TiC ceramic tool materials with all the additions discussed above were studied just for a few times [11, 13, 20, 21]. This paper focused on the influence of Ni addition on the mechanical properties and the microstructures of Al2O3–TiN–TiC ceramic composites. In the present work, the Al2O3–TiN–TiC ceramic materials with different amount of Ni addition were fabricated by hot-pressing technique. The mechanical properties such as flexural strength, Vickers hardness and fracture toughness were evaluated. The microstructures of the ceramic materials were observed. The effects of the content of Ni on the mechanical properties and the microstructures of Al2O3–TiN–TiC were investigated. The strengthening and toughening mechanisms were also discussed.
2.1 Starting Raw Materials and Sample Preparation
High-purity α-Al2O3 (99.9%) with the mean particle size of 0.5 μm was selected as the matrix material. Both TiC and TiN powders with the mean particle size of 0.5 μm were used as the reinforcing additives. MgO with the mean particle size of 0.5 μm was used as sintering aid. Mo and Ni powders with the mean particle size of 2.3 μm were chosen as the metal binders. Four material schemes were adopted according to the content of Ni addition. All the four materials had the same volume contents of TiC, TiN and MgO, which were 20, 10 and 1 vol%, respectively. The amount of Ni was ranged as 0, 1, 3 and 5 vol%, with the volume contents of Al2O3 were 68, 67, 65 and 63 vol%. The materials were numbered as ANC-Ni0, ANC-Ni1, ANC-Ni3 and ANC-Ni5.
The blended powders were ball milled using ethyl alcohol, and the mixture was homogenized for 48 h with alumina balls. Then the slurries were dried in a vacuum oven, and the powders to be hot pressed were made by sieving through a 100-mesh sieve. The final densification of the compacted powder was accomplished by hot pressing with a pressure of 32 MPa in vacuum. The sintering temperature employed for hot pressing was 1600 °C, and the holding time was 10 min.
Each sintering experiment was repeated at least three times to provide enough ceramic disks for the following work.
2.2 Characterization Techniques
The sintered disks were cut by a diamond inside diameter slicer (J5060C-1), and then the bars were ground and polished into rectangular bar specimens (4 × 3 × 30 mm). After chamfering the edges of the bars, the room temperature flexural strength experiment was conducted on a universal test machine (WDW-50E) by the three-point-bending test, the test span was 20 mm, and the loading speed was 0.5 mm/min. Twelve specimens with the same compositions were used. The flexural strength of each specimen was measured at least 6 times.
Measurements of Vickers hardness and fracture toughness were performed on the polished surface using a Vickers hardness testing machine (HVS-50). The load applied was 196 N, and the holding time was 15 s. The value of Vickers hardness was obtained with the formula (HV = 1.8544P/(2a)2). Here, P is the load value, and 2a is the diagonal length of the indentation. The indention produced median cracks around the indentation from which fracture toughness can be calculated with the function (KIC = 0.203HV (c/a)−3/2a1/2). Here, HV is the Vickers hardness, 2c is the trace length of the crack, and 2a is the diagonal length of the indentation. Both the Vickers hardness and the fracture toughness of each specimen were measured at least 10 times.
The microstructures of the polished and fracture surfaces were observed by scanning electron microscopy with an energy-dispersive spectrometry (SEM and EDS, ZEISS SUPRA-55).
3 Results and Discussion
Figure 2 shows the scanning electron microscope pictures of the fracture surfaces. The SEM micrographs indicated that there were many holes with large size in ANC-Ni0 (as indicated by the circles of Fig. 2a), which meant that the compactness was poor and it was detrimental to the mechanical properties. Compared to ANC-Ni0, the grain size of ANC-Ni1 was larger, the holes were less, the morphologies of transgranular fracture increased, and the cleavage steps on the surface were obvious (as shown in the circles of Fig. 2b). The transgranular fracture of the matrix grains could enhance the surface energy and hence improve the fracture toughness of the composites. The microstructure of ANC-Ni3 was homogeneous, and the grain size was much finer, which was beneficial to the mechanical properties. The fracture morphologies were mainly transgranular fracture, which meant the fracture toughness would be good. The grain size of ANC-Ni5 turned to be larger, there were strip interspaces, and the microstructure was inhomogeneous, which had unfavorable effects on flexural strength and fracture toughness.
In addition, there were intergranular fracture and transgranular fracture in all the four ceramic composites. With the increment of Ni addition, intergranular fracture reduced, whereas transgranular fracture increased. This can be explained that the increasing Ni weakened the grain boundary and the tensile stress of the matrix led to the decrease in the matrix strength, which made transgranular fracture easier to generate. The transgranular fracture can enhance the fracture toughness more effectively than intergranular fracture.
3.2 Mechanical Properties
The mechanical properties of the sintered materials
KIC (MPa m1/2)
All the sintered materials with Ni addition had higher flexural strength values than ANC-Ni0. The reinforcement phase TiC has fine solubility in Ni (the value can reach 11 wt% at 1400 °C), and Ni can promote surface strengthening and interface diffusion of TiC particles during liquid sintering process, so the materials with Ni addition have better flexural strength. The flexural strength first increased and then decreased with the increment of Ni content, when the Ni content was 3 vol%, the strengthen effect was the best. Figures 1c and 2c indicate that the grain size was small and uniform in ANC-Ni3, which could enhance the flexural strength according to Hall–Patch relationship. The pores of ANC-Ni5 were responsible for the decrease in flexural strength.
Same as the flexural strength, all the sintered materials with Ni addition had higher fracture toughness values than ANC-Ni0, and this indicated that Ni addition was beneficial for the improvement in fracture toughness. The fracture toughness of the Al2O3–TiN–TiC ceramic composites first increased and then decreased with the increment of Ni content, and the highest values also appeared when the Ni content was 3 vol%. It can be seen from Fig. 1c that the size of the ductile phase is large, the ductile nickel can stretch to failure between the crack faces, and as a result, the fracture toughness can be improved. The various core–shell structures can also enhance the fracture toughness. The microstructure of Fig. 2c also indicates that the cleavage fracture were beneficial for the improvement in fracture toughness. When the content of the Ni addition increased to 5 vol%, Fig. 2d shows that the transgranular fracture reduced which resulted in a decrease in the fracture toughness.
In summary, when the Ni content was 3 vol%, the mechanical properties of the ceramic materials were the best. Compared with the previous study about the effect of Mo addition on the Al2O3–TiN–TiC ceramic materials , at the same sintering conditions (32 MPa, 1600 °C, 10 min), the flexural strengths of Al2O3–TiN–TiC ceramic materials with Mo addition (617–850 MPa) are higher than those with Ni addition, which means that the addition of Mo can improve the flexural strength better than Ni addition. On the other hand, most fracture toughness values of ceramic materials with Ni addition are higher than those with Mo addition (6.05–6.46 MPa m1/2), which means that Ni can improve the fracture toughness of Al2O3–TiN–TiC ceramic materials more efficiently.
The introduction of the Ni particles into Al2O3–TiN–TiC ceramic materials had effects on both the mechanical properties and the microstructures. When the addition of Ni was 3 vol%, the flexural strength and fracture toughness were the best. The improvement in flexural strength could be explained by the refinement and homogeneity of the microstructure, and the improvement in fracture toughness could be explained by the various core–shell structures and the unique fracture mode.
It is proven that the effect of Ni as an effective additive is valid in Al2O3–TiN–TiC systems. The present results indicate that the ceramic materials are promising for machining applications considering further improvement in the hot-press progress.
This work was supported by the National Natural Science Foundation of China (50975161), the National Natural Science Foundation of China (51175305), the National Natural Science Foundation of Shandong Province (ZR2017BEE057), the Opening Laboratory Fund of Qufu Normal University (SK201516), and the Scientific Research Foundation of Qufu Normal University (BSQD20131426).
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