To evaluate which are the stable phases under high gas pressure conditions, a solid-gas reaction phase diagram under high gas pressure (HIP phase diagram) has been proposed by the author. The variables of the diagram are temperature, reactant gas partial pressure and total gas pressure. Up to the present time the diagrams have been constructed using isobaric conditions. In this work, the stable phases for a real HIP process were evaluated assuming an isochoric condition.

To understand the effect of the total gas pressure on stability is of primary importance: Two possibilities were considered and evaluated, those are: i) the total gas pressure acts as an independent variable, or ii) it only affects the fugacity values. The results of this work indicate that the total gas pressure acts as an independent variable, and in turn it also affects the fugacity values.

Silicon compounds with the (Mn5Si3)16H crystal structure in particular, have a high melting point, which is a necessary characteristic of high temperature structural materials. The compounds Si3Zr5 and Si3Ti2Zr3 with that crystal structure and with melting points around 2500 K, were densified by using a hot isostatic process (HIP) in a glass capsule. This crystal structure shows good creep resistance, which might be due to the particularly large silicon interatomic distance. The high melting point and good creep resistance, make these intermetallics good candidates for the next generation of high temperature structural materials.

Models of pressure-sintering and consolidation (such as that found in the HIP process) often assume a uniform spherical powder. However, it is known that use of mixtures, containing particles of differing composition and/or size, alters the initial powder packing and subseguent HIPing behavior. We have experimentally investigated these effects using the simplest form of mixture; a bi-modal powder with various size ratios and particle fractions. Packing experiments confirm the existence of “optimum” packing fractions for which the density is maximized over both monosized and the as-received (continuously distributed) powders. During HIPing these powder mixes remain relatively more dense than monosize packings. The consolidation of composite powder mixtures as well as microstructural evolution during HIP are also discussed. Results are compared with those predicted by modeling.

Removal of natural pore during capsule hot isostatic pressing (HIP) and normal sintering were examined directly with the novel liquid immersion technique, which can reveal details of micro-structure and characterize flaw-forming defects of microns-size in lightly densified specimens. The results were supplemented by the pore size measurement with the mercury porosimetry, microstructural examination with SEM as well as microstructural examination of specimens after final densification. The behaviors in both pore size and shape changes were different in HIP and normal sintering. Removal of large pores of microns-size started from the beginning of densification in HIP. Whereas large pores grow with densification in normal sintering. Large pores changes their shape drastically in HIP, but hardly changes in normal sintering. The behavior of pore in the early stage of densification is closely related to the microstructure of densified specimen; without HIP, large defects of processing origin are present in near-fully dense specimen. These observations were discussed in terms of current sintering theory.

This paper will discuss the role of Hot Isostatic Forging (HIF) of partially dense powder preforms to full density at greatly reduced temperatures at pressures to 60,000 psi. The floor to floor cycle times are minutes compared to hours for more conventional Hot Isostatic Processing (HIP) of materials. The process greatly reduces energy consumption and labor costs. Materials that would normally require temperatures of 1225°C to achieve full densification by HIP canbe densified at temperatures as low as 800 to 850°C. Composite materials can be processed rapidly enough at much lower temperatures by HIF to greatly reduce the potential for reaction between the matrix and the reinforcement material. In addition, composites can also be fabricated by a combination of SHS (Self- Propagating High Temperature Synthesis) and HIF to further reduce the reaction between the matrix and the reinforcement material. This permits going from a CIP'ed (Cold Isostatic Pressing) blend of Nickel, Aluminum, and Aluminum Oxide whiskers to final density in about 5 to 10 minutes. The process is amenable to the fabrication of near-net finished shapes by this rapid integrated cycle.

Sintering processes are monitored by dilatometry. A conventional dilatometer under high gas pressure (typical HIPping conditions) is constructed using graphite rods to examine linear thermal expansivity of one sample. This method has a few disadvantages, such as the pressure of graphite rods acting as a uniaxial hot press. The advantage of a HIP method, compared to a hot press sintering is the capacity to process many samples at a time. The graphite rod method is applicable for only one sample. A volumetric dilatometer was developed using buoyancy under high gas pressure, which can monitor HIP sintering processes of many samples and check for capsule failure.

New levitation materials processing on the ground was developed using high gas pressure. Conventional levitation materials processes are carried out in microgravity fields, such as in space or in free-falling, or by applying aerodynamic force, electromagnetic force etc.. This innovated method is based on buoyancy by very high density of high pressure gas. Advantages of this new method are not only as a containerless processing which is able to prepare high purity materials, to produce perfect spheres, but also the possibility to use normal HIP techniques. It is expected that the new processing is applicable in many fields and easy to commercialize.

Methods for surface treating and coating components by hot isostatic pressing are presented. It is shown that HIP'ing can improve the properties of conventional coatings and also be used to produce coatings that would be difficult to deposit by other techniques. The concept of using a combined coating and HIP treatment to modify surface properties by, for example, removing surface defects in metals and ceramics, is also discussed.

Thermal passivation of Si1−xGex (x=10 at.%) using dry high pressure (>700 atm) and hydrothermal (>500 atm) oxidation (HPO) was studied. Alloys of Si1−xGex approximately 200 nm thick were oxidized using three processes: (i) dry oxygen at 700 atm and at a temperature of 550°C, (ii) hydrothermal at 510 atm and 550°C, and (iii) conventional 1 atm at 800°C. Auger sputter depth profiling (AES), X-ray photoelectron spectroscopy (XPS), and Raman Spectroscopy were used to characterize the oxides. AES and Raman studies reveal that both dry high pressure and hydrothermal oxidizing conditions produce compositionally congruent oxides from Si90Ge10, multicomponent alloys. XPS studies were used to establish that the Ge in these oxides is chemically incorporated. Thermodynamic and kinetic considerations of the Si-Ge-O system are discussed for both HPO and conventional oxidation. The metastable nature of the HPO oxides is confirmed by examining the microstructure of annealed oxides.

Increasing the nitrogen concentration in iron and iron alloys significantly improves their mechanical properties. A recent technique for melting in a hot-isostatic pressure furnace using nitrogen as the pressurizing gas has been developed by U.S. Bureau of Mines researchers for making massive nitrogen additions to iron (up to 1.6 weight percent nitrogen) and iron-chromium-nickel alloys (up to 6.6 weight percent nitrogen). The total nitrogen concentration measured at atmospheric pressure and room temperature was determined to be the equilibrium nitrogen concentration in the molten alloy. Statistical correlations were derived to explain the effects of melt pressure and alloy composition on the resulting nitrogen concentration. Nitrogen concentrations measured in solidified alloys made by high pressure melting techniques at lower pressures are consistent with previous published data. Computer generated phase diagrams for high nitrogen-chromium concentrations are also consistent with nitride microstructure observed after high-pressure melting. Extension of existing atmospheric nitrogen concentration data to higher pressure nitrogen concentrations shows Sievert's law (nitrogen concentration is proportional to the square root of the nitrogen melt pressure) to be valid for pure iron. However, substantial deviations from Sievert's law are observed for higher alloy compositions. Statistical fits of thermodynamic concentration data to the high-pressure melt nitrogen data requires evaluating element concentration terms, interaction effect terms, pressure terms, and pressure-composition effects terms. Examination of the nitrogen concentration data suggests several methods of correlation.

In this paper the development of novel wear/corrosion resistant coatings for twin screw extruders used in the food industry is discussed. The conventional materials used for extruder applications are reviewed and their limitations highlighted. The requirements for a new surface engineering philosophy are introduced and utilised to develop a new manufacturing system for the coating of extruder barrels. These coatings are based on Ni/Cr/B/Si/Fe alloys containing WC particulate. The processing route combines conventional PM technology with consolidation of the coating by hot isostatic pressing (HIPping). The HIP cycle is tailored to optimise the coating/substrate adhesion and the coating microstructure. Examples are given which illustrate the superior performance of these HIPped bimetallic coatings compared to conventional surface treatments.

This paper describes the concept and the significance of atmosphere control in the HIP treatment of ceramics and reviews some practical methods to create desirable atmospheres during HIP treatment for some nitride and oxide ceramics by citing published experimental data. The authors propose new HIP equipment which enables atmosphere controlled HIP treatment involving a wide range of ceramics and compounds.

In situ reinforced (ISR) silicon nitride ceramics have been developed to have microstructures that mimic the best whisker containing ceramic matrix composites. Large, interlocking needle-like grains of beta silicon nitride can be produced throughout these materials to create an isotropic, high-temperature ceramic with high fracture toughness (˜9 MPa√m), good high-temperature strength (4 Pt MOR = 750 MPa at 25°C and 500 MPa at 1375°C), high Weibull modulus (m >20), and low creep at high temperature. Since these materials do not rely on transforming metastable phase inclusions as a toughening mechanism, their fracture resistance is virtually insensitive to temperature. The high crack growth resistance of these ceramics also yields a material which is extremely defect tolerant. Residual MOR strengths of 300–400 MPa are typical after multiple 50-kg Vicker's indentations of the sample tensile surface. After abrasive particle impact, the biaxial strengths of the in situ reinforced ceramics are typically more than twice that of traditional, fine-grained silicon nitrides.

Unlike ceramic composites toughened using whisker additives, the in situ reinforcement approach to silicon nitride development does not require the use of complicated whisker dispersion techniques for green processing, nor is shape-limiting hot pressing required for densification during sintering.

Open porous metals can be made by direct hot isostatic press (HIP) treatment on a cold isostatic pressed body. In this work, porous metals with uniaxially penetrating pores were produced using a modified HIP method.

Penetrating pores from one surface to the other surface are useful for filter applications, and were produced by forcing pressurized gas to travel through samples by alternating the HiPping pressure during sintering.

In this experiment, copper powder was used. Samples produced with this new method have higher permeability coefficient than those produced by the normal HIP method.

Production of porous alumina ceramics by capsule-free hot isostatic pressing of bimodally distributed alumina powders with a large size difference (>10) and reaction sintering of alumina powder and boron oxide formed by oxidation of boron nitride during sintering were investigated. The influence of the composition of bimodally distributed alumina and alumina-boron nitride mixture of powders on open and closed porosity was studied.

HIP treatment at 1600°C provided sintering of only fine-grained powder. A small loss in open porosity (about 4%) is observed with increasing of the fine powder weight fraction up to 0.4.

Reaction sintering of a fine-grained alumina powder with boron oxide gives significant (about 30%) increase of open porosity.

New processing of porous PZT ceramics with high connectivity was investigated using capsule-free O2 HIP technology. Porous PZT bodies were prepared by pressureless-sintering of green compacts with various amounts of spherical PMMA particles, followed by hot isostatically pressed at 1100°C and 200 MPa for 1 hour in 5%O2/Ar mixture gas atmosphere. The densification of porous PZT sintered bodies hardly occurred by capsule-free O2 treatment, resulting in the formation of well-developed neck growth between PZT grains. They exhibted a homogeneous microstructure with a bimodal pore size distribution consisting of both several ¼m and 50–80¼m pore sizes.

The capsule-free O2 HIPed porous PZT bodies have a larger permittivity and a smaller depolarizing factor about a half than the specimens before O2 -HIPing, suggesting that the HIPed PZT has a higher connectivity. Piezoelectric g33 constant-porosity relations were also described and mechanical property was discussed to indicate an increase of bending strength more than 20% compared to conventional sintering.

Open porous copper metals, which have high strength, high open porosity and well controlled pore size distribution, were produced by a hot isostatic press (HIP) process. They were sintered at different temperatures from 973 to 1273K under various HIPping pressures up to 200MPa. Pore size distribution and Young's modulus of the sintered samples were analyzed. The HIPped products have greater strength and higher open porosity than those of the normally sintered ones. The internal structural parameters such as pore size distribution were controlled by changing the HIPping pressure.

In 1965 Griggs and Blacic [1,2] proposed that there is a “hydrolytic weakening” process in quartz and silicates whereby the breaking of Si-O bonds, involved in the movement of dislocations, is facilitated by the presence of water. This proposal aimed to explain the observed dramatic weakening of quartz crystals when they are exposed to water in tests at high temperature, as well as the observed strong contrast in creep strength between dry natural quartz crystals and rapidly-grown synthetic quartz crystals containing traces of water. Such a “hydrolytic” process may also underlie the observed effects of water in accelerating other phenomena such as self-diffusion of oxygen in quartz, aluminium-silicon ordering in feldspars, slow crack propagation in silicates, and recrystallization in quartz. A review of this field is given in Reference 3.

Fiber dispersed porous alumina ceramics were produced by a direct high gas pressure sintering method. The produced materials exhibited relative densities of 75–85%, high bending strength and good thermal shock resistance. The alumina ceramics were characterized by lower closed porosity and higher open porosity. These characters strengthen the thermal shock resistance of porous ceramics. This high gas pressure sintering by the direct high gas pressure sintering method densifies only bridge parts of alumina ceramics which support pores. This process is concluded to be effective to produce materials with higher strength, better thermal shock resistance and higher open porosity.

Porous silicon nitride ceramics with open porosity fraction of about 0.25 were obtained by capsule-free hot isostatic pressing from as-received and aqueous Soxlet washed powders. Particle surface modification by aqueous washing increases green density of cold isostatically pressed bodies and leads to a decrease in open pore fraction. At the same time, the α–β transition rate of the treated powder decreases.