EXPLORING THE PROCESS TO OBTAIN A COMPOSITE BASED ON Cr2O3–AlN USING A METHOD OF HOT PRESSING

Modern commercially-available types of cutting ceramics largely meet the requirements to the process of high-efficiency dry blade machining of the difficult-to-treat materials based on Fe, Ni, Co, Cr, W, etc. The functional properties of such materials are ensured in the first place by a high level of physical-chemical and mechanical properties of the starting micro-powders. In addition, the powders must be physically and chemically inert to the high-temperature intramolecular interaction with a machined material, and resistant to oxidation in the process of cutting. At present, these criteria are met by a limited number of micro-powders from refractory compounds, in particular, oxides of aluminum and zirconium (partially stabilized), titanium carbide and carbonitride, and silicon nitride. Proof of this is that 20 major companies from Sweden, Japan, the United States, Germany, Russia, China, South Korea, etc. industrially produce those cutting materials that were designed 30‒40 years ago, four kinds of ceramics: white (Al2O3), mixed (Al2O3–TiC, TiN), nitride (Si3N4-based), fibrous (Al2O3–SiCW). The technology of obtaining conventional cutting ceramics employs a costly and low-efficient operation of hot-pressing that involves graphite tooling. High (>1,600 °C) temperature and pressure (30– 40 MPa) greatly increase energy intensity and reduce the environmental friendliness of a given technology. Attempts to use much more expensive and non-technological nano-powders from refractory compounds [1–3] based on the conventional production technology of cutting plates failed to enhance their quality though increased the cost [4, 5]. Therefore, in terms of materials science, the base for designing competitive cutting ceramic materials should be the improvement of quality and the reduction of energy intensity of the HP process, specifically by decreasing its temperature and pressure. One of such fields is the technology of reactive synthesis [6]. EXPLORING THE PROCESS TO OBTAIN A COMPOSITE BASED ON Cr2O3–AlN USING A METHOD OF HOT PRESSING


Introduction
Modern commercially-available types of cutting ceramics largely meet the requirements to the process of high-efficiency dry blade machining of the difficult-to-treat materials based on Fe, Ni, Co, Cr, W, etc. The functional properties of such materials are ensured in the first place by a high level of physical-chemical and mechanical properties of the starting micro-powders. In addition, the powders must be physically and chemically inert to the high-temperature intramolecular interaction with a machined material, and resistant to oxidation in the process of cutting. At present, these criteria are met by a limited number of micro-powders from refractory compounds, in particular, oxides of aluminum and zirconium (partially stabilized), titanium carbide and carbonitride, and silicon nitride. Proof of this is that 20 major companies from Sweden, Japan, the United States, Germany, Russia, China, South Korea, etc. industrially produce those cutting materials that were designed 30-40 years ago, four kinds of ceramics: white (Al 2 O 3 ), mixed (Al 2 O 3 -TiC, TiN), nitride (Si 3 N 4 -based), fibrous (Al 2 O 3 -SiC W ). The technology of obtaining conventional cutting ceramics employs a costly and low-efficient operation of hot-pressing that involves graphite tooling. High (>1,600 °C) temperature and pressure (30-40 MPa) greatly increase energy intensity and reduce the environmental friendliness of a given technology. Attempts to use much more expensive and non-technological nano-powders from refractory compounds [1][2][3] based on the conventional production technology of cutting plates failed to enhance their quality though increased the cost [4,5].
Therefore, in terms of materials science, the base for designing competitive cutting ceramic materials should be the improvement of quality and the reduction of energy intensity of the HP process, specifically by decreasing its temperature and pressure. One of such fields is the technology of reactive synthesis [6].

Literature review and problem statement
One of the implemented technologies of reactive synthesis is the development of the material "Bichromit-R" based on a mixture of the micro-powder Cr 2 О 3 and 15 (% by weight) of the micro-powder AlN [6]. According to [7], in the process of HP of the mixture Cr 2 О 3 -15AlN nano at a temperature of 1 600 °C and a pressure of 18 MPa, as a result of the exothermic reaction among its components, there forms a new matrix phase from the solid solution Cr x Al 1-x ) 2 О 3 of various composition and the phases of CrN, Cr 2 N dispersed in it. The peculiarity of a given technology is the use of a cheap commercially-available powder of chromium oxide as a basic component of the resulting mixture.
Study [8] has shown that the stability of Bichromit-R plates under the finishing machining of hardened steel of grade ShKh15 (HRC60) is at the level of stability for plates made from the mixed ceramics of grade CC650 by Sandvik Coromant (Sweden). High operating properties are explained by the authors are due to a higher crack resistance (Mises theory), as compared to pure aluminum oxide in conventional ceramics, of the newly formed matrix phase of solid solution based on Cr 2 О 3 and Al 2 О 3. This could be a consequence of the coherence of the newly formed phasesthe planes [ [7,9]. This structural factor dominates over the presence in it of brittle and non-strong dispersed inclusions from Cr 2 N.
The main disadvantage of a given technology is still its high energy intensity due to the temperature and pressure that are similar to the technology of obtaining conventional ceramics. That became one of the main reasons for the absence of Bichromit-R in the market of modern instrumental composite materials. The key issue that was not resolved when designing a given material was the lack of studies into the influence of heating rate, specifically between 1,250-1,500 °C, at which the intense compaction of the material occurs, as well as the chemical interaction between Cr 2 О 3 and AlN nano [9].
This factor is very important for the rapid-proceeding, and typically self-emerging, exothermic reactions in powder mixtures, especially when they are compacted under the influence of thermobaric parameters of the HP process.
In addition, when exploring the process of obtaining Bichromit-R, given the lack of necessary instruments, the quantitative content of nitrogen and carbon in its structure was not investigated. That did not make it possible for the authors to define the actual chemical composition of its components.
It is known that CrN forms from elements when heated to 1,200 °C. Therefore, it is of scientific and practical interest to investigate the influence of heating rate and different pressure at HP of the mixture Cr 2 О 3 -15AlN nano on the process of its compaction, structure formation, and the properties of the material.

The aim and objectives of the study
The aim of this study is to investigate the compaction, structure formation, and the properties of the material at hot pressing of the mixture Cr 2 О 3 with the nanopowder AlN at different pressures and heating rates.
To accomplish the aim, the following tasks have been set: -to explore the impact of heating rate at HP of the mixture Cr 2 О 3 -15AlN nano on the process of compaction, structure formation, and the properties of the material; -to define the conditions for the formation of the material with maximum hardness and crack resistance; -to perform the micro-roentgen spectral study of the chemical composition of the components of the structure of the obtained material with maximal mechanical properties.

Materials and methods to study the microstructure and properties of a cutting material
The study was carried out using the following powders: -chromium oxide powder, grade "pure", GOST 2912-79; -aluminum nitride, grade "pure", from furnace synthesis, TU6-09-110-75, made at Donetsk plant "Chemical reagents" (Ukraine); -the nanopowder AlN, from plasma chemical synthesis (the city of Riga) with specific surface S=20-40 m 2 /g (specifications from the manufacturer).
For the study, we prepared mixtures of Cr 2 О 3 with 15 AlN (an analog to the mixture Bichromit-R). Hereafter, % by weight.
A mixture of powders with alcohol was stirred in an iron drum by milling bodies made from the alloy BK at a ball mill over 24 hours. The hot pressing of the mixture was carried out in an open air in a graphite mold at the installation with an induction current heating of frequency 2.4 kHz.
Parameters for the process of HP for a billet with a diameter of 20 mm: temperature T=1,520 °C, exposure 10 min, pressure on punches P=8 and 18 MPa. We heated the mold to 1,000 °C at a rate of 60-70 °C/min. In the range of 1,000-1,520 °C, the heating of the mixture was performed at a rate of 150, 350, and 600 °C/min in order to determine the maximum influence of the exothermic reaction of the charge components on its compaction, as well as on the properties of the material. The temperature of 1,520 °C for HP was taken based on a condition that it exceeds the temperature (1,450 °C) of the intensive chemical interaction between Cr 2 О 3 and AlN nano , and it is close (1,600 °C) to the temperature for obtaining Bichromit-R [6].
One batch of samples was heated under a constant pressure of 8 MPa, then the pressure was increased to 18 MPa. The pressure of 18 MPa corresponds to the pressure that was used in the process of HP of Bichromit-R. The pressure of 8 MPa was taken based on the point of view about the possible additional impact of effect of the reaction on the HP process. A stepwise increase in the pressure from 8 to 18 MPa at 1,100 °C for the second case is due to the insufficient strength of the graphite mold. Cooling of a sample following the isothermal exposure was carried out at a simultaneous decrease in the temperature to 800 °C and the loadto 0 over 15 min.
We explored the microstructure and chemical composition at REM (ZEISS, JENA, Germany). It should be noted that the finely grained, multi-phase structure, especially if obtained from a chemical synthesis, is a difficult object to study the chemical composition of its components. Fig. 1 shows the distribution diagrams of dimensions for the micro-powder Cr 2 О 3 . Images in Fig. 2 show that the size of powders of Cr 2 О 3 in the original state (Fig. 2, a) are roughly in the range of (0.7-1.5 µm). Fig. 3 shows compaction diagrams of the mixture 15AlN for different, p=8 and 18 MPa, pressure on punches.  For a heating rate of 600 °C/min the compaction curves, regardless of the mixture composition, for pressures of 8 and 18 MPa are similar in shape. This indicates that at a heating rate of 600 °C/min the process of the mixture compaction, in addition to pressure, is additionally activated by the effect of exothermic reaction among its components.

Results of studying the process of hot pressing
Dependence diagrams of hardness and crack resistance of the material 15АlN nano on heating rate are shown in Fig. 4. Increase the heating rate from 150 to 600 °C/min increases the hardness and crack resistance by, respectively, 1.0 GPa and 1.5 MPa•m 1/2 . In this case, the maximum values for hardness and crack resistance of the examined material are by 0.3 and 0.4 units larger than those for its analog Bichromit-R [5]. Fig. 4 shows that this value for crack resistance in diagram 3 is matched with a value for a heating rate of 400 °C/min. That was probably the rate at which they heated the mixture at HP of the material Bichromit-R, which was not registered in [5].
We studied the microstructure (Fig. 5) using samples with the maximum values for hardness and crack resistance, obtained under a pressure of 8 MPa and at the maximum (600 °C/min) heating rate (Fig. 3). Based on the results from a local chemical analysis, Table 1 gives the share ratios of components in the matrix and disperse phases of the examined samples, as well as the content of impurities in them. Fig. 5 shows that the basic structure of the examined sample is of the dispersed-strengthened type. The matrix phase is the solid solution of variable composition (Cr х Al 1-x ) 2 O 3 (0<x<0.5) newly formed in the process of reaction. Dispersed inclusions (of light color) are stochastically distributed throughout the matrix phase. By the ratio of N and Cr content, they represent Cr 2 N. In addition, they revealed up to 1.8 % of Al. Separate inclusions also demonstrated up to 1.53 % of oxygen.
A feature of the structure of samples (Fig. 5, c) is the presence of individual macro-structural formations of different rounded shapes in it, whose dimensions are in the range of 10-30 µm. In these inclusions, the matrix phase (point T1, Fig. 5, c) is darker in color than the matrix phase (point T2, Fig. 5, c) of the basic structure. Its chemical composition corresponds to the component of the solid solution Cr 1-x Al x ) 2 О 3 , (0.5<x<1.0).
That is, it contains more aluminum and less Cr than in the solid solution of the basic structure. Table 1 Results from chemical analysis of components of the structure of the material 15AlN, obtained at GP under a pressure of 8 MPa and at a heating rate of 600 °C/min The dispersed particles based on Cr 2 N in a given macro-inclusion have the same composition of chemical elements, while 2 times smaller size, than the inclusions to the basic structure (Fig. 5, c and Table 1). At the interface between a given inclusion and the basic structure the dispersed particles based on Cr 2 N have mostly an elongated shape and the enlarged size (2-10 µm). It is also important to note that the microhardness HV(0.5 Н) of the material inside these macro-formations is 1.03 times greater than the microhardness of the material beyond its border.
The disadvantage of a given structure of the material is the presence of separate individual inclusions based on chrome nitride the size of up to 10 µm (point T1 in Fig. 5, a, and point T4, Fig. 5, b). Fig. 6 shows fractograms of the 15AlN samples. The surface of the samples' fracture ( Fig. 6, a) has a clearly expressed relief with rounded ridges, non-characteristic of the conventional ceramics. It is also important to note the presence at the fracture's surface of the micropores of spherical shape smaller than 1 µm. One can see that the destruction of a given sample occurs mainly in line with the intercrystallite mechanism. A small share of flat facets testifies to the presence of trans-crystallite destruction as well. Similar relief of the fracture's surface is also demonstrated by highly-viscous materials based on complex compounds, such as type of МАХ-phase.

Discussion of results of studying the process of hot pressing
The structure of the dispersed type, formed in the HP process of the reactive mixture Cr 2 O 3 -15AlN nano is characteristic of the modern ceramic materials for instrumental purposes. The intensification of compaction process of the mixture Cr 2 O 3 -AlN with an increase in heating rate from 150 to 600 °C in the range of 800-1520 °C with a decrease in pressure from 18 to 8 MPa during HP directly points to the influence of exothermal effect due to the interaction between its components. Formation of solid solutions of alternating composition of the type (Al 2-х Cr х ) 2 O 3 and the disperse phase in it based on Cr 2 N that contains up to 1.8 % Al is consistent with the results reported in [10]. The presence of macro-structural formations of the composite type ( Fig. 5, a, b) with the chemical composition of the matrix phase, the size and content of dispersed inclusions, different from the base structure, is explained by the agglomerated character of the AlN nanopowder in the starting mixture. The mechanism of their formations is the result of the penetration of gaseous and liquid products from the disintegration of chromium oxide to the pores of aggregates from nanoparticles of aluminum nitride.
The presence of up to 1.5 % of oxygen in separate chromium nitride inclusions (Table 1) is probably the result of that the x-ray beam overlapped the solid solution. The absence of CrN in the structure of a material, compared to the analog Bichromit-R, is due to the high rate of heating at which aluminum nitride cannot form at a temperature of 800-1,100 °C.
It is known that Cr 2 N in terms of a set of properties is not included in the group of compounds for instrumental purposes [10]. Therefore, the high fracture toughness of the obtained material is explained as follows: -formation of the matrix phase of solid solution with complex alternating composition (Cr 1-x -Al x ) 2 О 3 ; -formation of dispersed inclusions of solid solution up to 1.8 % Al in Cr 2 N; -formation of individual structural macro-formations of the rounded shape the size of up to 30 µm with a matrix phase of the type (Al 1-x -Cr х ) 2 О 3. The presence in the structure of the obtained material of separate large (up to 10 µm) inclusions based on Cr 2 N, may lead, by analogy with the inclusions of this size to the structure of conventional ceramics, to a decrease in its crack resistance.

Conclusions
1. We have established the effect of increasing the heating rate, from 150 to 600 °C/min, on the process at HP of the mixture Cr 2 O 3 -15AlN nano , specifically: -it reduces pressure of the complete compaction of a material from 18 to 8, resulting in lower energy intensity of the technology; -it improves the hardness and fracture toughness by, respectively, 1.0 GPa and 1.5 MPa•m 1/2 .
2. It has been shown that the structure of the obtained material with maximum hardness (HV18.7 GPa) and crack resistance (K 1C = MPa•m 1/2 ) is a structure of the dispersed type -the matrix phase of solid solution (Cr 1-x -Al x ) 2 О 3 and the particles of Cr 2 N, dispersed in it, with inclusions to 1.8 % Al.
3. In terms of structural characteristics, chemical composition, hardness and crack resistance, the material obtained in this work meets modern requirements to instrumental composite material for high-efficiency finishing and semi-finishing machining of difficult-to-treat materials (HRC 65) (carbon, alloyed steels, cast iron, refractory alloys, etc.).