The work of modern materials assumes operation under extreme load and temperature conditions and often in very unfavorable environments. This creates the need to use new materials with higher strength parameters or modify the properties of traditional materials to meet these expectations.
The growing expectations of customers and strong competition also play a major role, which makes it necessary for entrepreneurs to increase the efficiency of production and adapt some of the products by designing them according to the needs of a particular customer. This, in turn, often involves attempts to introduce new technological solutions to the market, which will be able to meet the needs of the most demanding applications and thus give an advantage on the market.
In the case of materials, currently the most possibilities are related to shaping the structure of the material on a nanometric scale. This is ensured by solutions based on powder consolidation with the use of technology using electric impulses for sintering. The SPS (Spark Plasma Sintering) technology enables sintering of materials at lower temperatures compared to other solutions, which significantly reduces or even eliminates grain growth in the material. This technology has been known for many years, but only now, mainly due to the possibilities of modern technology, it shows its extraordinary abilities in the design and manufacture of modern materials, which are suitable for the most demanding applications.
The term “Spark Plasma Sintering” is commonly used, but at the same time it is quite confusing because neither the spark nor the plasma are present in the process, which has been experimentally confirmed. In this regard, using the name “Field Assisted Sintering” is more appropriate.
The main characteristic of FAST is that the pulsed DC current passes directly through the graphite matrix and, in the case of conductive samples, also through the powder. This allows the material to be heated with joule heat, which, unlike traditional sintering techniques, allows a density close to or equal to the theoretical density at a much lower sintering temperature. This is because heat is generated inside the material and is not transferred indirectly through heating elements as in conventional hot pressing.
This solution allows to achieve a very high heating rate of up to 1000℃/min, which allows for a very fast sintering process, which lasts from a few to several minutes depending on the material and sample diameter. Such a fast process ensures that powders are compacted by means of nano-grids or nano-structures while avoiding thickening that occurs with conventional methods of sintering. This makes FAST technology a very good method for obtaining materials based on nanoparticles.
GeniCore has developed a FAST sintering device that owes its name U-FAST (Upgraded Field Assisted Sintering Technology) to, among other things, an improved power supply system that enables the generation of current pulses of less than one millisecond. This allows to produce materials in a more efficient and economical way compared to competitive methods.
Nowadays, it is the only such solution in the world, which also allows for very precise determination of the value of energy supplied to the material and thus for very precise process control.
This is reflected in a very visible way in the results, as evidenced by the characteristics of materials produced by this method. It is also important that the shaping of materials takes place without pre-pressing, isostatic compacting and drying.
Examples of materials made of U-FAST
U-FAST sintering technology, as one of the most modern techniques of powder material consolidation, can be used to produce a wide range of materials. These include:
- Sintered carbide (WCO) is used, among others, in the cutting tools industry, as a material for powder pressing dies. This is due to the combination of desired properties such as high hardness and mechanical strength. Carbide cutting elements are usually produced by such methods as hot pressing (Hot Pressing, HP) and hot isostatic pressing (Hot Isostatic Pressing, HIP). However, these methods have many disadvantages, such as low speed and long sintering time, high equipment costs and usage. From this point of view, sintering with FAST techniques is a more advantageous solution.
Tungsten carbide was sintered as an example. The sintering of the WC was made of powder with an average grain size of 0.65 μm, containing inhibitors and with the addition of 10% cobalt.
The material with hardness of 1800 HV and density of 99.8-100% compared to the theoretical density was obtained by conducting processes at temperatures of 1100-1220°C. The obtained material is characterized by high uniformity of microstructure. The grains after the sintering process did not undergo any unfavorable growth, as shown in the figure below.
It is known from the literature that materials with such a high density are only obtained by using a WC without inhibitors as an output powder. The disadvantage of not using such additives is unfavorable grain growth. So far, from inhibited WC powder it has been possible to obtain a material with a density of only slightly more than 99%, in the case of 10 nm WC granulation (Materials and Manufacturing Processes, 30: 327-334, 2015), which remains much lower than in the case of materials manufactured with U-FAST.
The GeniCore device is equipped with a pyrometer operating in the full range of RT-2500℃, so it is possible to produce materials that require low temperatures. Examples are thermoelectric materials or metallic glasses.
- Thermoelectrics (PbTe) – Thermoelectric technologies involve the direct conversion of waste heat into electricity and vice versa. Thermoelectric generators are used, among other things, to convert energy in “silent” atomic-powered submarines, cooling of network transformers or air-conditioning system modules, they work as microgenerators supplying sensor systems or Peltier elements cooling microprocessors. PbTe compounds are popular thermoelectric materials, thanks to many desirable features they have, such as isotropic morphology, high crystalline symmetry, low thermal conductivity, ability to control the concentration of media.
Sintering of lead telluride was performed by conducting processes in 5 minutes at 400°C. Depending on the powder preparation conditions, materials with a density in the range of 98.8-100% of theoretical density were obtained. This parameter translates into favourable thermoelectric properties, as thermal and electrical conductivity, as well as Seebeck factor, are strongly dependent on the microstructure of the material.
- Zr-Cu-Al-Nb (Bulk Metallic Glasses) are available in a wide range of layouts. Compared to conventional metals and alloys, they have better mechanical and chemical properties due to their amorphous structure. Therefore, they are considered to be a new generation of materials very resistant to stress. In addition, they have a very high tensile yield strength (1.9 GPa), elastic deformation (2%), hardness (600 HV) and corrosion resistance. However, BMGs are difficult to produce due to the relatively high cooling rates required for amorphous solidification. Metallic glasses can be used in many fields of technology, for example in the form of foam for the construction of future spacecraft for long-term flight into space or in medicine.
Sintering in Zr-Cu-Al-Nb system was carried out from the powder mixture. As a result, after the process at 420°C, a material of more than 99% theoretical density was obtained. It is characterized by homogeneity of the microstructure, and most importantly, XRD measurements did not show the presence of crystalline phase.
It is known from the literature that sintering processes using another device in the same arrangement have not yet managed to produce an amorphous material with a density close to the theoretical value (Journal of Alloys and Compounds, vol. 667, 2016, pp. 109-114).
- Composite materials are currently the most numerous group which are ceramic-ceramic, ceramic-metal, metal-metal composites, as well as their combinations with polymers. New materials are being searched for, which would meet the constantly growing requirements of parameters, and thus new areas of application are being discovered. On the U-FAST device various composites can be successfully produced, regardless of their classification.
- Tungsten-silver composites characterized by high resistance to arc erosion while having high resistance to clamping. They are used for electrical contacts in many different applications such as electrical switches, contactors, circuit breakers, voltage regulators, arc terminals, relays. This material is produced by infiltrating tungsten frame or powder metallurgy, but then additional rolling is used to increase the density. Commercially available in 50W-50Ag version has a density of 13.15 g/cm3 (http://www.stanfordmaterials.com/Tungsten-silver.html).
Tungsten sintering with the addition of silver in a 50/50 ratio was performed. The processes were carried out at 900°C. The obtained material has a hardness of 1.0 GPa and a density of 13.40 g/cm3, exceeding the density of the material available on the market. What is more, the manufacturing process consists of filling the matrix with a mixture of tungsten and silver powders and sintering, without additional stages, infiltration or rolling.
- Composite materials in the zirconium oxide (TZP) – aluminum oxide (α-Al2O3) system show good mechanical properties (strength, hardness) and very good abrasive wear under various environmental conditions. They are used as universal structural ceramics in many technical applications. Al2O3 and ZrO2 are materials with low reactivity compared to metals or polymers and are therefore used as bioceramics. Such composites are non-toxic, do not cause allergies, which combined with excellent frictional properties makes them ceramics used in endoprosthesis.
Using the U-FAST technology, the composite was produced in a 1:1 volumetric ratio. Sintering at 1400°C resulted in a material with a homogenous microstructure and clear phase penetration. As can be seen in the SEM image, the technique used allows to keep the size of the batch powder grains, which is not possible with other sintering methods. This composite has a density of 99%, hardness close to 18 GPa and resistance to brittle fracture 9.5 MPa・m0.5.
- Ceramic materials, which due to their corrosion resistance, ability to work at high temperatures and mechanical properties, are attractive structural and functional materials of great importance in technology.
Aluminum has many interesting properties such as high strength, high hardness and excellent corrosion resistance. Thanks to this, it can be used in a transparent form, for example, as an electromagnetic window, transparent armor, cover of metal halide lamps. Sintered Al2O3 ceramics with submicron grain size is the hardest material of all transparent materials. Therefore, it is considered that transparent polycrystalline aluminum oxide is a promising alternative to sapphire. Traditionally, it is produced by sintering in hydrogen at temperatures above 1700°C.
Ceramic translucent material produced in U-FAST technology is characterized by a high degree of sintering, 99.9% theoretical density and HV 2220 hardness. In the wavelength range 3.5-5 µm the transmission value is min. The transmission value is at least 80%, which is similar to that of commercially available crystals. The sintering process was carried out at temperatures in the range of 1100-1200°C, in a classic graphite set, using a commercially available high-purity aluminum oxide powder. No additional technological stage was used which would increase the cost of production.
- Biomaterials. The research (Biomedical Journal of Scientific & Technical Research, 10.26717/BJSTR.2019.23.003952) has resulted in a perfectly concentrated (100% theoretical density) titanium material, a popular alloy and composites. Mainly due to its biocompatibility, corrosion resistance in body fluids, titanium is used in medicine. It is used for making implants, stents and dentures. Nevertheless, the main disadvantage of titanium, in addition to high production costs, is its relatively low resistance to wear. Composites with titanium-metal matrix (TMMC) used to eliminate this disadvantage are increasingly common.
U-FAST sintering technology allowed to obtain (Ti,Mo) C/C materials at 1050°C during sintering for 2 minutes. The obtained materials have a hardness of 1580 HV, which is almost 800% higher than that of pure titanium. U-FAST technology is therefore suitable for the production of titanium-based biomaterials and may contribute to the creation of materials with even more favorable parameters.