Core/rim microstructure of Ti(C, N) cermets with low nickel-molybdenum binder content

Introduction

The KNT series of tungsten-free hard alloys (TFHA) are composed of titanium carbonitride Ti(C, N) and nickel and molybdenum powders as fusible binders. These alloys can be used as an alternative to tungsten carbide hard alloys. The KNT alloys exhibit unique properties that distinguish them from other TFHAs including high hardness and low specific gravity. For example, the widely used KNT16 alloy (GOST 26530-85) has a hardness of 89 HRA and a density of 5.9 g/ cm³ [1–4].

The titanium carbonitride-based cermet offers several advantages, such as its easy availability and simple manufacturing. Moreover it exhibit higher hardness at high temperatures compared to tungsten carbide, along with superior scale resistance. Additionally, a thin oxide film is formed on the surface of the cermet during tool operates at elevated temperatures, which acts as a lubricant. As a result, the KNT alloys exhibit low friction coefficient and good wear resistance. Nonetheless, KNT alloys also possess certain drawbacks, including low impact toughness and thermal conductivity, and a high coefficient of thermal expansion. Consequently, these properties increase the likelihood of cracking when the tool is soldered and sharpened [2].

We developed a TFHA (KNT series) alloy with a reduced amount of No–Mo bonder, to evaluate its suitability as carbide cores for armor-piercing projectiles [3; 5]. These alloy has improved ceramic properties due the presence of a metallic bond improves their [6–12]. The authors presented the sintering conditions, essential TFHA microstructure characteristics, and physical and mechanical properties of the KNT3 and KNT7 alloys [13–15].

The microstructure of the KNT alloy is characterized by a core/rim structure (CRS) [15–20]. The core is comprised of a permanent liquid phase (PLP) consisting of carbonitride TiC_1 – xNi_x, while the rim (which forms the shell of the core) is a multicomponent carbonitride (Ti, Mo)(C, N). The formation of the cermet microstructure is primarily attributed to the wetting of the solid phase by the molten binder, facilitated by a wetting angle close to zero at the interface between the solid phase and the melt.

The objective of this study is to examine the impact of the carbon-to-nitrogen ratio in the permanent liquid phase (PLP) on the formation of the core/rim structure (CRS).

Alloys with a low binder content

Figure 1 presents composite contrast SEM cross sections of KNT7 and KNT3 alloys. It is apparent from the Fig. 1 that the coaxial shell of the base metal grains, or the rim, around the cermet cores in KNT3 occupies a larger surface area compared to KNT7. This notable difference in the microstructures of KNT3 cermets and conventional hard alloys was previously by Pakholkov V. et al. [3]. The authors suggested variations in the manufacturing process conditions such as sintering temperature and time, as shown in the table, can lead to the formation of different microstructures in cases where the content of the melt liquid phase is inadequate.

The rim (shell) formation by the dissolution-sedimentation reaction [4] is limited by the amount of available molybdenum in the liquid phase. The shell formation may be associated with molybdenum solid-phase mass transfer over the interphase interfaces. There are no available studies of this phenomenon so we decided to investigate the CRS phase and structure formation during the interaction between titanium carbonitride and metallic melts at various stages of sintering.

Formation of CRS ctructure in KNT alloys
with a low binder content

The detailed synthesis of KNT3 and KNT7 cermets is presented in references [3; 13]. The values of sintering temperature (t_sn) and isothermal holding time (τ_h) for each sample (see the Table) can be attributed to the increased contribution of solid-phase sintering to cermet formation as the metal component content decreases.

The permanent liquid phase in KNT alloys is titanium carbonitride, which has a double crystal structure consisting of TiC and TiN. Its thermodynamic compatibility with each metal component of the binder phase varies. Cermets are primarily composed on carbon + metal compounds. Nitrogen interacts with metals only at high temperatures and may or may not form weak nitrides. For example, titanium nitride (TiN) is used as an intermediate buffer layer in the electronics industry. It serves as an effective barrier for preventing the diffusion flows between conductive contact components.

The synthesis of the KNT alloy involves three stages of sintering, namely heating, holding at the melting temperature, and cooling in the furnace.

During the heating stage, several reactions take place between the refractory components and the refractory and binder components. These processes occur partially before the liquid phase appears and include gassing, diffusion reactions, and shrinkage of the powder compacts. As the powder mixture is heated, CO is released starting at approximately 900 °С and reaches its maximum release rate at around 1100 °С. Nitrogen is released at ~1200 °С and reaches its maximum release rate at 1300 °С. The nitrogen release rate decreases at temperatures above 1300 °С is associated with the beginning of CRS growth in cermets.

During the the low-temperature sintering phase, the rubber-based plasticizer (4–5 % gasoline solution) [21; 22] is removed before the liquid metal (melt) emerges. Upon decomposition, the plasticizer yields the Mo₂C molybdenum compound, which may account for another characteristic of the shell microstructure. Figure 2 illustrates the two-layer shell structure of the KNT3 cermet, comprising an inner shell consisting of a solid solution rich in heavy elements, that surrounds the Ti(C, N) grain; and an outer shell is Ti-based material.

The last stage of the two-level shell microstructure formation, particularly its onset, remains ambiguous and conflicting. One proposal suggests that the final shell structure is created during the final stage of sintering, whereby TiC and MoC carbide are deposited on the Ti(C, N) particles. These carbides form a solid solution dissolved in the liquid binder. Another proposal assumes that the inner shell is formed by a solid-phase interaction at the initial sintering stage (up to 900 °C), while the outer shell is formed by dissolution/deposition.

The second sintering stage begins with the interaction between the Ni and Mo binders and refractory phases and the emergence of the liquid phase. The dissolution/deposition process becomes a significant contributor to the formation of the sintered cermet microstructure.

According to available sources, during the sintering process, Ti(C, N) reacts with the melt such that the liquid molybdenum facilitates the dissolution of titanium and carbon from the PLP, while the nitrogen-rich, poorly soluble carbonitride remains preserved as a solid phase. The enrichment of the refractory phase with titanium nitride is more or less pronounced depending on the liquid/solid phase ratio. The nitrogen-rich areas of the Ti(C, N) grains remain insoluble in the liquid metal bond and act as crystallization nuclei of the (Ti, Mo)C carbide solutions deposited from the melt by dissolution/sedimentation, leading to the formation of the so-called K-phase [4].

The formation of the K-phase takes place during the liquid-phase sintering of hard alloys in the presence of a carbide-forming element in the melt. As the liquid phase emerges, the TiC component of the PLP begins to dissolve, forming (Ti, Mo)C. The deposition of this compound is possible only when the limit solubility product value Mo_{1 – n}Ti_n C_y is reached.

The core size of the Ti(C, N) particles increases with the sintering time primarily due to particle coalescence at their interfaces, which is more intense under liquid-phase sintering than under solid-phase sintering, before the shell structure formation. The rate of shell deposition on the Ti(C, N) grains depends on the sintering temperature and (Ti, Mo)C concentration in the melt. The shell thickness reaches 0.5–3.0 μm as the sintering temperature increases from 1450 to 1540 °С.

Pakholkov V. et al. [3] observed that as the volume fraction of the Ni–Mo binder decreases, the degree of coalescence of the Ti(C, N) grains increases. The specific volume content (V_V) of the Ti shell (rim) consisting of Ti_1 – xMo_xC_yN_z exhibits an inverse relationship between the V_V of the shell (as shown in the table) and binder volume, but the reason for this is unclear.

The furnace cooling rate is controlled and does not exceed 10 °C/min to ensure a smooth temperature gradient across the TFHA thickness, thus tavoiding thermal cracking caused by different coefficients of linear thermal expansion of the TFHA components. The shell formation by dissolution-deposition from the melt (Ti_1 – nMo_n)C_x in the KNT3 alloy is limited by the volume content of the binder. At the final stage of the CRS formation, significant shell growth can be attributed to a solid-phase molybdenum mass transfer across the TiC_xN_z–(Ti_1 – nMo_n)C_x interface [3]. Figure 3 illustrates the CRS difference for KNT7 and KNT3 cermets for identical sintering conditions (t_sn = 1560 °C; τ = 60 min).

Conclusions

We have determined that the synthesis of KNT solid alloy can be divided into three stages: heating, holding at the temperature required for melt formation, and cooling in the furnace. Our conclusion can be summarized as follows.

1. We have observed that the decrease in volume fraction of the Ni–Mo metallic binder leads to an increase in the volume fraction of the Ti_1 – xMo_xC_y N_z .

2. The absence of nitrogen-enriched areas in the Ti(C, N) grains leads to an increase in the diffusion rate of molybdenum across the refractory phase interfaces during the cooling stage.

3. We have discovered a two-level structure of the cermet shell with inner and outer layers. The inner shell (appearing as light areas in the SEM image) is molybdenum-rich, while the outer shell is Ti-rich (appearing as dark gray areas).

4. We propose a chemical explanation for the formation of the inner cermet shell as a result of the decomposition of the rubber-based plasticizer (4–5 % gasoline solution) during the heating stage.