the boiling point of tantalum hafnium carbide
Investigating the highest melting temperature materials
TaC, HfC, and their solid solutions are promising candidate materials for thermal protection structures in hypersonic vehicles because of their very high melting temperatures (>4000 K) among other properties.
The melting temperatures of slightly hypostoichiometric TaC, HfC, and three solid solution compositions (Ta1−xHfxC, with x = 0.8, 0.5, and 0.2) have long been identified as the highest known.
In the current research, they were reassessed, for the first time in the last fifty years, using a laser heating technique.
They were found to melt in the range of 4041–4232 K, with HfC having the highest and TaC the lowest.
Spectral radiance of the hot samples was measured in situ, showing that the optical emissivity of these compounds plays a fundamental role in their heat balance.
Independently, the results show that the melting point for HfC0.98, (4232 ± 84) K, is the highest recorded for any compound studied until now.
The design of next-generation hypersonic flight vehicles has raised interest in the discovery and development of materials that can operate in extreme environments.
Hypersonic vehicles are equipped with sharp nose tips and leading edges to maximize flight performance.
However, very high temperatures and heating rates are produced at these surfaces due to extreme velocities (>5 Mach).
Thermal protection structures are required that can operate in the air at temperatures that can exceed 2200 K, thus components are required to have very high melting temperatures 1, 2.
The extreme conditions required for hypersonic applications have introduced the motivation for research and development of high-temperature materials, including a group of ceramics commonly known as ultra-high temperature ceramics (UHTCs).
A common definition of a UHTC is that of a ceramic that has a melting temperature above 3300 K.
From a wide selection of materials that an engineer can choose only a limited number have melting temperatures above this criterion 3.
Tantalum carbide (TaC) and hafnium carbide (HfC) are of particular interest due to their high melting temperatures (>4000 K) which are the highest reported among all known inorganic materials 1, 2, 3.
They are known to form a continuous solid solution over the whole range of compositions.
The measurement of thermophysical properties at such high temperatures is difficult due to increased reactivity of the materials, heat losses, volatility, and loss of mechanical strength, particularly in the carbides of interest in this work.
Several experimental investigations of the high-temperature behavior of pure and mixed tantalum and hafnium carbides were carried out mostly between 1930 and 1969, including the early work by Agte and Althertum 4 and the comprehensive experimental campaign provided by Rudy 5.
In all these cases, the high-temperature melting of these carbides was studied with the help of the so-called Pirani-Alterthum approach 4.
This experimental method consists in observing the disappearance, in a sample heated by Joule effect, of a blackbody hole when it is filled by a newly formed liquid.
Such an approach has been shown to be effective in many cases.
However, it does involve significant uncertainty, especially at temperatures above 2200 K, where observation of liquid formation is extremely hard, in particular when using the early pyrometers that had limited time resolution.
Agate and Altherthum 4 reported a maximum melting temperature (Tm) for Ta0.8Hf0.2C of 4213 K, and this was later confirmed by Andriievski et al.
6 who measured it as 4263 K.
The temperature trend with a maximum melting point at an intermediate composition was not confirmed in the work by Rudy 7.
Rudy reported that the highest melting temperature compound is TaC0.88 at 4256 K, with a decreasing trend in Tm as the HfC concentration increased and a Tm of 4201 K for HfC0.94.
In addition, the work by Gusev 8 reports a calculated phase diagram (CALPHAD) for the TaC-HfC system in which the melting temperatures of the single-member carbides are higher than those of the solid solutions, with TaC as the highest melting compound at 4275 K and 55 K higher than HfC.
More recently, Hong and van de Walle 9 predicted the melting temperatures using density functional theory (DFT), and their calculations suggest that the melting temperature of HfC0.81 is 3962 K, higher than TaC0.88 at 3830 K.
Also these calculations were consistent with a local maximum within the solid solutions, for a composition close to Ta0.8Hf0.2C at 3920 K.
The work of Hong and de Walle was particularly useful, in that it identified precise physical mechanisms behind the effects of carbon hypostoichiometry and alloying on the melting temperature of a given composition.
In addition, HfC0.56N0.38 was reported as the compound with the highest melting temperature at 4141 K.
In summary, the uncertain and contradictory results on the melting points reported suggest that: (i) thanks to the entropic contribution of lattice defects to reducing the free energy of the solid phase, slightly hypostoichiometric monocarbides might have melting temperatures higher than their solid solutions 7, 8, 9 ;
(ii) because of the Fermi energy position in the mixed carbides, a maximum melting temperature within the solid solutions (TaC-HfC) might exist 4, 6, 9.
This calls for further experimental work perhaps with other techniques than were previously available.
The laser melting technique used in this work constitutes an alternative to the Pirani-Alterthum method for studying the melting temperatures of the Ta-Hf-C compounds.
It allows precise control of the time for which the sample is kept at extreme temperatures, which can be reached in the order of milliseconds if need be.
Experiments of subsecond duration address several of the challenges associated with measurements at very high temperatures.
The laser pulse shape and duration can be optimized in order to produce the desired heating and cooling rates while avoiding or at least minimizing undesired effects such as sample vaporization and reaction with the container.
A pressure cell filled with an inert buffer gas at a moderate pressure of about 0.3 MPa is used to slow down the evaporation from the sample surface and prevent the coating of the windows.
Millimetre long graphite screws hold the sample, so that interaction with the container is kept to a minimum.
In addition, only the central region of the sample surface is typically melted.
This is then surrounded by solid material in both the radial and axial directions.
The surrounding solid part of the sample can be considered to form a kind of “self-crucible” insulating the area of interest from the sample holder and preventing contamination with foreign material.
A fast pyrometer with a time resolution of the order of 10 μs and spectro-pyrometer with a time resolution of the order of 1 ms is used to record the sample thermal radiance in the visible and near-infrared spectral range.
The laser melting technique has been successfully used to study several refractory systems such as uranium carbides 10, uranium oxides 11, plutonium oxides 12, 13, uranium nitrides 14, and zirconium carbides 15, 16.
The results obtained on compounds with already well-assessed phase transition temperatures are in good agreement with the literature data.
With the progress of the industrial world, there has been a need to continuously develop and improve cutting tool materials and their geometry.
Cutting tools are often of interest when manufacturers look for improvements in overall productivity.
Technologies such as high-speed machining and dry machining place extreme demands on cutting tools.
Since different machining applications require different cutting tool materials, many types of tool materials, ranging from high carbon steel to ceramics and diamonds, are used as cutting tools in the current metalworking industry.
It is important to be aware of the fact that differences do exist among tool materials, as schematically presented in Fig. 14.2 (Kalpakjian and Schmid, 2006).
The following section will briefly discuss the most important types of cutting materials and their different properties and applications.
Schematic representation of the difference in properties of tool materials.
HIP, hot isostatic pressing;
HSS, high strength steel.
14.2.1 Cemented carbides
Cemented (or sintered) carbides, known collectively in most parts of the world as hard metals, are a range of very hard, refractory, wear-resistant alloys made by powder metallurgy techniques.
The minute carbide or nitride particles are cemented by a binder metal that is liquid at the sintering temperature.
Early pieces of cemented carbide were too brittle for industrial use, but it was soon discovered that mixing tungsten carbide powder with up to 10% of metals such as iron, nickel or cobalt allowed pressed compacts to be sintered at about 1500 °C to gain a product with low porosity, very high hardness and considerable strength (Jones et al., 2004).
Tungsten carbide (WC) is the most common hard phase, and cobalt (Co) alloy is the most common binder phase.
These two materials form the basis for cemented carbide structures and grades.
Other types of cemented carbide have been developed.
In addition to these simple WC–Co compositions, cemented carbides may contain varying proportions of titanium carbide (TiC), tantalum carbide (TaC) or niobium carbide (NbC) and others.
Cemented carbides, which have the cobalt binder phase alloyed with, or completely replaced by, other metals such as nickel, chromium, iron, molybdenum or alloys of these elements, are also produced.
This combination of materials makes them ideal for use as metal cutting tools (Sandvik, n.d.).
In many cases, for tools for finish machining (especially the turning of special alloys), usually a TiC-based cermet with Ni and Ni (Mo) alloys or a steel binder is used.
In spite of Ni and Ni alloys being the most widely used binder materials for TiC cermets, FeNi iron alloys are attractive.
Cermets based on TiC are often used because of properties such as low density, relatively high mechanical strength, high oxidation resistance, and a low (close to steel) thermal expansion coefficient.
Titanium carbide cermets with a steel binder are used as materials for high-cycle fatigue-stamping and as blanking or die materials.
A key feature of this material is the potential to vary its composition so that the resulting physical and chemical properties ensure maximum resistance to wear, deformation, fracture, corrosion, and oxidation.
As this material is expensive and has low rupture strengths, it is normally used in the form of tips, which are brazed or clamped to a steel shank.
Carbide inserts coated with wear-resistant compounds perform well and have a long tool life and are the fastest-growing segment of the cutting tool materials spectrum.
The use of coated carbide inserts has permitted a significant increase in machining rates in comparison to the rates possible with uncoated carbide tools.
The first coated insert consisted of a thin titanium carbide layer on a conventional WC substrate and was made using a chemical vapor deposition (CVD) process.
Ever since, various single and multiple coatings of carbides and nitrides of titanium, hafnium, zirconium, and coatings of oxides of aluminum and zirconium have been developed to increase the range of applications for coated carbide inserts (Cubberly and Bakerjian, 1989).
The TiC–Al2O3 and TiC–Al2O3–TiN-coated tools are generally employed at higher speeds than the TiC–TiCN–TiN-coated tools.
When machining steels at higher speeds, the alumina layers provide excellent crater resistance.
These grades work well on both irons and steels but are not recommended for materials such as aluminum or titanium (Davis, 1995).
The physical vapor deposition (PVD) coating process, commonly used on high-speed steel tools, was tested on carbide and consistently produced a damage-free interface.
Subsequent cutting tests with PVD coated tools confirmed that an increase of the tool life could be observed when compared with CVD coated tools.
A special group of PVD coated carbides is recommended for heavy or interrupted cutting applications (Whitney, 1994).
These inserts are available in a wide variety of chip control geometries in all standard insert configurations.
There is little doubt that the coated carbides come closest to an all-purpose cutting tool material for ferrous alloys.
Their success is the reason why coated carbides account for approximately 60% of all sales of indexable inserts.
Ceramic tools are far superior to sintered carbides in respect of hot hardness, chemical stability and resistance to heat and wear, but are lacking in fracture toughness and strength.
They are well suited for machining cast iron, hard steels, and superalloys.
Two types of ceramic cutting tools are available: alumina-based and silicon nitride-based ceramics.
The alumina-based ceramics are used for highspeed semi- and final finishing of ferrous and some non-ferrous materials.
The silicon nitride-based ceramics are generally used for rougher and heavier machining of cast iron and superalloys.
In the last few years, remarkable improvements in strength and toughness, and hence the overall performance of ceramic tools, have become possible by several means, which include (Whitney, 1994):
•Sinterability, microstructure, strength, and toughness of Al2O3 ceramics were improved to some extent by adding TiO2 and MgO.
•Transformation toughening by adding an appropriate amount of partially or fully stabilized zirconia to Al2O3 powder.
•Isostatic and hot isostatic pressing (HIP).
•Introducing nitride ceramic (Si3N4) with a proper sintering technique – this material is very tough but prone to built-up-edge formation when machining steels.
•Adding a carbide like TiC (5–15%) to Al2O3 powder – to impart toughness and thermal conductivity.
•Reinforcing oxide or nitride ceramics with SiC whiskers, which enhances the strength, toughness, and life of the tool and thus spectacularly increases productivity.
•Toughening Al2O3 ceramic by adding a suitable metal like silver, which also imparts thermal conductivity and self-lubricating properties;
this novel and inexpensive tool are still in an experimental stage.
Cutting tools made from industrial grade and mined single-crystal diamonds have been used for many years.
However, many applications use polycrystalline diamond tools.
Polycrystalline diamond (PCD) tools consist of fine diamond crystals compacted and bonded together under high pressure and temperature.
Usually the diamond is bonded to a standard carbide insert as a single tip on the insert.
The insert is then ground (and perhaps polished) to give a very smooth finish on the diamond.
A PCD tool is an excellent general-purpose tool for machining non-ferrous and non-metallic, abrasive materials (Rufe, 2002).
When polycrystalline diamond blanks are bonded to a tungsten or tungsten carbide substrate, cutting tools are produced that are not only high in hardness and abrasion resistance but also greater in strength and shock resistance (Cubberly and Bakerjian, 1989).
The most common applications for PCD tools include copper and aluminum alloys, machined at high cutting speeds.
It is standard practice for aluminum automobile wheels.
Another common application for PCD tools is the machining of non-metallics such as hard reinforced plastics and materials such as granite and marble.
Because PCD is much more abrasion resistant than carbide, it can be used at higher cutting speeds and/or has a longer tool life than carbides for the same machining operation.
14.2.5 Cubic boron nitride
Next to diamond, cubic boron nitride (CBN) is the hardest material presently available.
Tools are available as both tipped inserts (like PCD) and also in solid CBN inserts.
The solid inserts cost about three times as much as the tipped inserts but offer multiple cutting edges and much tougher cutting material.
The extreme hardness, toughness, chemical, and thermal stability, and wear resistance of CBN are the advantages that make it suitable for high material removal rates as well as precision machining, imparting excellent surface integrity to the products.
Such unique tools are effectively and beneficially used in machining a wide range of work materials, covering high carbon and alloy steels, non-ferrous metals and alloys, exotic metals like Niard, Inconel Nimonic, and many non-metallic materials, which are difficult to machine with conventional tools.
It is firmly stable at temperatures up to 1400 °C.
CBN must compete with lower-priced ceramic tools for cutting steels in the 55–63 Rockwell Hardness (HRC) range.
A fine alumina/TiC ceramic is about 10% the price of a single-tipped CBN insert and can have four to eight cutting edges.
It is wise to consider the entire economic situation of any machining operation in order to justify high-performance and high-price cutting tool materials such as CBN.
A machining operation where solid CBN inserts are better than other tool materials is face milling high-strength steels (more than 60 HRC).
Solid CBN inserts have an incredible toughness and, when correctly applied, can withstand the most severe interruptions without chipping and breaking (Whitney, 1994).
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