**1. Introduction**

Quickly developing industry and need for materials of better utility parameters contribute to development of innovative production technologies. Industry is always in search of new technologies which meet growing demands of faster better and cheaper products. Powder metallurgy is modern technology of manufacturing of ceramic, metallic and composite products. Commonly applied technologies of the powder metallurgy based on the sintering process include following stages:

	- **•** particles of the powder come close at a distance allowing adhesion,
	- **•** increase of contact surface area of the powder due to relocation and their plastic deformation,
	- **•** peeling the oxide coating off through abrasion of the neighbouring particles,
	- **•** and local point sintering of the particles due to plastic deformation and increase of temperature.

**iv.** Processing of the sinters; the final stage after which the product is ready to use.

The sintering process itself is a process depending on transforming of the powder material into new stable material with different physical properties. The characteristic feature of the technology is possibility of serial production of ready elements and semi-products of the highest quality and strictly specified composition, without impurities and faults connected with conventional technological procedures of plastic processing or foundry. Besides, parsi‐ mony in the use of materials, energy, time and automation of the process make the method very economical and prospective. Therefore, in recent years, the interest in modern sintering methods in the production processes has increased [1-4].

Sintering is the process involving many quantitative and qualitative factors. The first group of the factors affecting the sintering are values characterizing a preformed product from a power, such as: particle size of the powder, its chemical and phase composition, density of a profile, inhomogeneity of the profile. On the other hand, the second group of the factors make parameters controlled during the sintering process such as: temperature, time, pressure, sintering atmosphere. It is also to remember about so called random factors, which, in uncontrolled way, may influence the result of sintering, e.g. inhomogeneous chemical composition or inhomogeneity of mixtures [1].

From the mentioned above three groups of factors influencing properties and microstructure of the sintered materials, the most important are the following:


acceleration of diffusion processes, formation of transient liquid phase and prevent grain growth [1,4-8].

**iv.** Processing of the sinters; the final stage after which the product is ready to use.

methods in the production processes has increased [1-4].

126 Sintering Techniques of Materials

composition or inhomogeneity of mixtures [1].

sintering can be shorter.

etc.

of the sintered materials, the most important are the following:

a material and limits also the process of grains growth.

The sintering process itself is a process depending on transforming of the powder material into new stable material with different physical properties. The characteristic feature of the technology is possibility of serial production of ready elements and semi-products of the highest quality and strictly specified composition, without impurities and faults connected with conventional technological procedures of plastic processing or foundry. Besides, parsi‐ mony in the use of materials, energy, time and automation of the process make the method very economical and prospective. Therefore, in recent years, the interest in modern sintering

Sintering is the process involving many quantitative and qualitative factors. The first group of the factors affecting the sintering are values characterizing a preformed product from a power, such as: particle size of the powder, its chemical and phase composition, density of a profile, inhomogeneity of the profile. On the other hand, the second group of the factors make parameters controlled during the sintering process such as: temperature, time, pressure, sintering atmosphere. It is also to remember about so called random factors, which, in uncontrolled way, may influence the result of sintering, e.g. inhomogeneous chemical

From the mentioned above three groups of factors influencing properties and microstructure

**•** Temperature of sintering, which should ensure successful finalizing consolidation processes and receiving of required utility properties. It should be selected in such a way, that during

**•** Time of sintering is the time during which the sintering material is kept at the sintering temperature. Carefully chosen time allows for effective course of diffusion mechanisms in

**•** Pressure; application of the external load causes intensification of the consolidation processes and, among others, simplifies the elimination process of pores at the final stages of the sintering. Due to application of pressure, temperature can be lowered and time of

**•** Atmosphere of sintering; during the sintering process, neutral (protective) or active atmosphere can be applied. In most cases the process is realized in the neutral atmosphere, whose main advantage is protection of the sintering materials again oxidation. This kind of atmospheres are selected in such a way, so possible reaction with a sinter is forbidden. On the other hand, the active atmospheres directly influence the sintering process causing desirable changes of the chemical composition, *eg.* prevent decomposition of the compound before finish of the sintering or reduce oxides present in the powder that impede sintering

**•** Activators of sintering are used in order to accelerate contraction, decrease sintering temperature or formation of proper microstructure of a sinter. They influence mainly

consolidation, diffusion mechanisms causing solidifying contraction dominated.

Among techniques limiting grain growth and accelerating consolidation are sintering under pressure (Hot Isostatic Pressing (HIP) and High Pressure – High Temperature (HP-HT) or microwave sintering or the one taking advantage of current impulses. The main advantage of the sintering by HP-HT method is opportunity to achieve extremely high pressures together with high temperatures during the process. It is worth noting, that under influence of simul‐ taneous operation of pressure and temperature, the process proceeds much faster (usually time does not exceed few minutes) than in the case of free sintering (which usually lasts from few to several hours). The obtained sinters are characterized by a degree of densification reaching almost 100% and isotropic properties. The use of such conditions can also reduce the diffusion of particles and prevent grain growth. The high-pressure devices are composed of hydraulic presses and specially-designed chambers that permit sintering. Presently, many solutions for such high-pressure chambers exist. Basic types of the chambers applied in industry are the following: spherical chambers of Bridgman type, 'Belt' type chambers and multi-die cubic chambers. Such construction solutions of the synthesis chamber allow for relatively large volume of reaction charge, optimal distribution of pressure and achievement of high sintering temperature. Their characteristic feature is attaining quasi-hydrostatic state of stress through stable medium transferring pressure, such as various kinds of rocks, most often. The highest possible temperature at which the sintering process can be carried by means of HP-HT method is 2000°C, or even more – depending on the duration of the sintering process [9-11]. The HP-HT method is applied to sintering large group of materials, for example: diamond [10], cubic boron nitride (cBN) [12], TiB2 ceramics [13], gradient materials [14,15], composite materials [12,16-19] and others.

In recent years, many papers were published on new techniques of sintering, in which heating of charge is performed by impulse current source. Taylor is recognized as a progenitor of this technology [20], who in 1933 proposed a pioneering solution involving to heat up during sintering Joule heat released during current flow through the consolidating powder. This process was named resistance sintering (RS). Currently, large interest in this kind of sintering is connected with its technical and economical advantages. The resistance sintering allows to obtain dense sinters with good properties at lower temperature and short time (from few to several minutes) [21]. The RS techniques involving pulsed current include Pulse Plasma Sintering (PPS) and Spark Plasma Sintering (SPS) methods. The SPS method was developed in 1960 initially to sinter metal powders [22]. However, due to high cost of equipment and low efficiency of the sintering process, the solution has not found its application at the beginning. Only in 80's of the 20th century, a new generation of apparatus for sintering of materials was elaborated under a name Spark Plasma Sintering. The SPS method was used even then to produce modern composite and functional gradient materials [23,24]. Up till now, not all phenomena associated with pulsed current sintering have been explained. There are many theories regarding the SPS sintering. Yoshimura *et al.* proved experimentally [25] that consol‐ idation intensifies processes caused by the pulsed current.

**Figure 1.** A schematic drawing of the pulsed current that flows through powder particles [26]

Currently accepted concept basis on the phenomenon of electric discharge. Heating up of the sintered materials is caused by pulsed current that may flow in two ways: either through graphite die and punches or through press powder particles. At the moment of current flow through powder particles, electric discharge occurs at places, where particles touch each other and at free spaces in the material (Figure 1). As a result, in these regions, a momentary increase of temperature even up to several dozen thousands degrees of Celsius is observed. The heat in the process concentrates at surface of the particles. Then, vaporizing occurs followed by cleaning, activation of the sample surface and increase of diffusion processes on the surface and grain boundaries. Next, partial melting takes place and formation of necks between particles being connected [27-29]. The pulsed current during SPS sintering is usually applied during consolidation of materials that conduct electric current. On the other hand, insulators must be consolidated with the application of the direct current. Main advantages of the SPS method are as follows:


Thanks to SPS method, sinters of the following materials were produced in the recent years: metals and their alloys (eg. Fe, Cu, Al, Au, Ag, Ni, Cr, Ti, Mo etc.), oxide ceramics (eg. Al2O3, MgO, ZrO2, TiO2, SiO2), carbides (eg. SiC, B4C, TiC, ZrC, WC), nitrides (eg. TiN. Si2N4, TaN, ZrN, AlN), borides (eg. TiB2, ZrB2, HfB2, VB2), fluorides (eg. LiF, CaF2), composite materials and intermetallic phases [24, 25, 31-35].

Currently, the TiB2 compound belongs to the group of modern engineering ceramics. The interest in this material has been developing since the beginning of 2000. Previously, the use of TiB2 was limited for technological reasons. TiB2 ceramics is characterised by a unique combination of physico-chemical properties that allow it to be used under the conditions of high temperature and in corrosive environments. The most attractive properties of TiB2 are:


**Figure 1.** A schematic drawing of the pulsed current that flows through powder particles [26]

**•** shortening of the time of sintering, what prohibits the grain grow of materials,

**•** elimination of application of the sintering agents in the case of many materials,

**•** faster and better purifying and activation of the powder particle surfaces, what enhances

**•** consolidation of materials without the application of the initial pressing, isostatic densifi‐

Thanks to SPS method, sinters of the following materials were produced in the recent years: metals and their alloys (eg. Fe, Cu, Al, Au, Ag, Ni, Cr, Ti, Mo etc.), oxide ceramics (eg. Al2O3,

**•** enabling fast densification under relatively low temperatures,

method are as follows:

128 Sintering Techniques of Materials

sintering activity,

cation and drying [26,29,30].

Currently accepted concept basis on the phenomenon of electric discharge. Heating up of the sintered materials is caused by pulsed current that may flow in two ways: either through graphite die and punches or through press powder particles. At the moment of current flow through powder particles, electric discharge occurs at places, where particles touch each other and at free spaces in the material (Figure 1). As a result, in these regions, a momentary increase of temperature even up to several dozen thousands degrees of Celsius is observed. The heat in the process concentrates at surface of the particles. Then, vaporizing occurs followed by cleaning, activation of the sample surface and increase of diffusion processes on the surface and grain boundaries. Next, partial melting takes place and formation of necks between particles being connected [27-29]. The pulsed current during SPS sintering is usually applied during consolidation of materials that conduct electric current. On the other hand, insulators must be consolidated with the application of the direct current. Main advantages of the SPS

**•** low density (4.5-4.62 g/cm2 ).

Titanium diboride is also characterised by a very good resistance to oxidation, chemical and structural stability at high temperatures, thermal shock resistance and abrasion resistance [36-38]. Currently, TiB2 ceramics is an attractive material for specific applications in the aviation, automotive, defence, and aerospace industries, including the production of armours for land vehicles, ships and planes, aerospace parts and parts operating at high temperatures, characterised by very good abrasion resistance. Restrictions on more extensive use of TiB2 ceramics are mainly due to difficulties associated with the sintering process and obtaining in this way a pure ceramic material of suitable density. They include, among others:


Despite these difficulties of purely technological character, recent years have brought an increased interest in titanium diboride, with focus on its use as a phase reinforcing the metal matrix composites. Technical literature [41-46] presents studies that relate to the use of TiB2 ceramics as a reinforcing phase of composites based on, among others, iron, aluminium, copper, titanium, or cobalt, and on their respective alloys. Authors of the research works focus their attention mainly on studies of the effect of the amount of the reinforcing phase on the properties and microstructure of these composites and on the development of best technology for their manufacture. In the composites reinforced with particles of TiB2, an important issue is to choose the right size of the reinforcing phase and at the same time its optimum percent content in the composite material. Depending on the volume fraction and particle size of each material, sintered products with highly varied microstructure and properties can be obtained. Studies described in [48,49] show the impact of TiB2 ceramics on microstructure and tribolog‐ ical as well as mechanical properties of austenitic stainless steel sintered by HIP. Nahme et al. [47] studied the mechanical properties of AISI 316L stainless steel reinforced with 15 vol% of TiB2, including its behaviour at elevated temperatures. An improvement was obtained in the Young's modulus (218 GPa), tensile strength (885 MPa) and compression strength (1800 MPa) of the sintered materials. As proved by various research works, the deformation of sintered AISI 316L stainless steel reinforced with 15 vol% of TiB2 was significantly reduced from 45% to 6%. Microstructural observations revealed a uniform distribution of TiB2 ceramics in the examined material and presence of phases rich in Cr/Mo in places where the grains of this ceramics appeared. Tjong et al. [48] examined the properties of AISI 304 austenitic stainless steel reinforced with varying amounts of TiB2 ceramics. It has been shown that increasing the content of the reinforcing TiB2 phase improved both hardness and tensile strength, but at the expense of reduced ductility. Based on the tribological tests carried out, a dramatic improve‐ ment of abrasion resistance has been obtained with increased volume fraction of the ceramic phase.

The aim of the presented study was to analysis of the effect of sintering techniques on the physical, mechanical and tribological properties of 316L austenitic steel-TiB2 composites.
