Hot-Press Furnace Systems Handle Extreme Operating Conditions
Hot press furnace systems are used to process ceramic, ceramic/metal matrix, and intermetallic composites. Typical applications include diffusion-bonding studies; hot compacting of oxides, nitrides, borides, carbides, sulfides and their mixtures to near theoretical densities; and sintering ceramics and powder metals. A hot-press furnace system typically is capable of operating temperatures from 500 to 2500 C (930 to 4530 F), up to a maximum temperature of 3000 C (5430 F). Because of the extremely high operating temperatures, system design is critical to ensure safe, reliable operation and long service life.
Furnace system design criteria include type of atmosphere in which the load is processed, including inert gas, such as helium, argon and hydrogen and reactive (reducing) gases and vacuum partial pressure. The system also includes a mechanical vacuum pump to evacuate and backfill with process gas or to maintain an operating vacuum level of 10-2 torr. High vacuum pumps are available for an operating vacuum level up to 10-10 torr. The heat zone design is based on temperature rating. Generally, three major components make up a hot press furnace system including a controlled-atmosphere furnace, hydraulic press system and vacuum pumping and process-gas control systems.
The controlled-atmosphere furnace consists of a furnace chamber, heat zone, power supply package and control instrumentation cabinet. The chamber assembly is a double-wall, water-cooled configuration with water-cooled full opening, hinged door access.
The heat zone is the most vital component of the furnace, consisting of heating elements, insulation or heat shields, and a water jacket for additional cooling if operating temperatures exceed 2000 C (3630 F). Proper selection of element, insulation and heat shield materials (based on temperature and process requirements) is critical for proper operation, as shown in Table 1. Maximum operating temperature varies depending on hot zone materials and process gas. The compatibility of these combinations is shown in Table 2.
Geometry of the heat zone and heating-element location also are critical to proper operation. For example, a cross section of a furnace designed with graphite heat shields and graphite heating elements is shown in Fig. 1. In this instance, the heat zone is significantly higher than the load to allow free radiation from the heating elements to the top and bottom sides of the load. Figure 2 shows a metallic hot zone with mesh-type heating elements.
Graphite heat shields are single layer, rigid board insulation. Metallic heat shields are multilayered sheets separated by a spacer. Specific to Oxy-Gon's design are the use of 0.25 mm (0.010 in.) thick sheet for all layers. Shields are precisely and stably separated using helix spacers made of 2.9 mm wide x 0.25 mm thick (0.115 in. x 0.010 in) ribbon. Traditionally used coiled wire spacers change diameter when stretched.
Heating-element voltage is controlled via a power supply package that is able to adjust for the sharp increase in electrical resistance of the element with increasing temperature. At maximum temperature, heating elements typically operate at approximately 20 V. Line voltage is converted to operating voltage via a transformer, and a phase-angle silicon controlled rectifier (SCR) controls voltage to the primary side of the power transformer.
The control instrument cabinet also serves as the central process control center for the entire hot press furnace system. A microprocessor based multiloop, programmable, process controller is used to control temperature of the heat zone and the force generated by the hydraulic press system.
A fixed thermocouple senses temperature up to 2000 C (3630 F) and is retracted from the heat zone above 2000 C, upon which an optical, infrared pyrometer is engaged to sense the heat zone temperature. The closed-loop control system provides fully proportional control of temperature through the entire operating range. The programmable process controller also provides fully proportional control of force through the entire cycle via load cell.
An independent over-temperature protection instrument signals the contactor located in the power supply package to disengage and remove power from the heating elements when an over-temperature condition is sensed.
An independent vacuum controller, incorporated in the control instrumentation cabinet, senses pressure with thermocouple gauges below 10-3 torr and an ion gauge from 10-3 to 10-10 torr pressure.
The final major component of the control instrumentation cabinet is a programmable logic controller (PLC). Function of the PLC is to operate all remaining electro-mechanical devices.
Hydraulic press system
Figure 3 shows a typical hydraulic circuit and path of force through the system. The use of a servo control valve provides fully proportional control of pressure throughout the entire process cycle. A linear encoder is capable of measuring movement within +/-10 microns accuracy.
Atmosphere conditions within the furnace chamber are controlled to prevent oxidation of the load and heat zone components. The conditions are neutral such as with nitrogen and argon, reducing with hydrogen and other reactive gases and vacuum.
A mechanical, or roughing, pump is used to evacuate and backfill the furnace chamber with process gas or operate at a vacuum level of approximately 10-2 torr, the practical operating limit of a mechanical pumping system. Four or five evacuation and backfill cycles typically are required to reduce residual oxygen to an acceptable level when operating with process gas. Mechanical pumps are also used to evacuate discharge of certain molecular high vacuum pumps.
Molecular type pumping systems are used to achieve operating vacuum levels higher than 10-2 torr. These types of pumps use several different means to physically "knock" molecules out of the environment, such as diffusion, turbo molecular and cryopumps, which absorbs or captures molecules using a helium refrigeration system (Fig. 4). Ultimate vacuum capability of these pumps is higher than operating vacuum levels. Operating vacuum level is an objective value determined to be reasonable and achievable for most hot-press furnace applications. A typical high vacuum system is illustrated in Fig. 5.
In the process of achieving a vacuum, the foreline is evacuated through the foreline valve while the high vacuum, and roughing valves remain closed. When an appropriate foreline vacuum level has been established, the high-vacuum pump is initiated and stabilized. The foreline valve closes and the roughing valve opens, and the chamber is evacuated by the roughing pump through a port in the high-vacuum valve. The right-angle, high-vacuum valve remains closed to the high-vacuum pump. When the chamber reaches an acceptable vacuum level, and contingent on the foreline maintaining its vacuum level, the roughing valve closes and the high-vacuum valve opens to the furnace chamber. The system is now operating in high vacuum mode, and the vacuum level is sensed by an ion gage. Foreline pressure is constantly monitored by a thermocouple gage and initiates a system alarm should foreline pressure exceed a safe minimum.
Additional atmosphere control system components can include a burn-off stack, flame arrester, and relief port for use with hydrogen or other combustible gases. Condensation traps are used when the load outgases heavy hydrocarbons such as wax used for binder material.
Putting technology to work
Busek Co. Inc., Natick, Mass., manufactures critical parts for use in aerospace and military applications including hot pressing and sintering ceramic powders in a graphite die at 1500 C (2730 F) and metallizing various configurations of ceramic parts. The company also conducts research and development of microfluidic devices. The devices consist of micromachined (0.025 mm, or 0.001 in.) capillary quartz plates approximately 50 mm wide x 50 mm deep x 3 mm thick (approx. 2 in. x 2 in. x 0.12 in.) diffusion bonded to form a leak-tight structure between the plates while maintaining capillary integrity. Other plates are 50 mm wide x 50 mm deep x 1 mm thick (2 x 2 x 0.04 in.) gold-plated stainless steel, micromachined to the same capillary tolerance. Diffusion bonding is done under pressure at 1050 C (1920 F). Metal components are processed under pressure at 800 C (1470 F). Both material matrixes are bonded at a vacuum level of 10-5 torr. The pumping system consists of a turbo mechanical high vacuum pump backed with a mechanical roughing pump.
To produce these and other parts, Busek required a small hot-press, controlled atmosphere furnace of a conventional cold-wall design with a refractory metal heat zone for use in a laboratory environment. Process requirements dictated that the system could handle diffusion bonding and press applications at temperatures to 2000 C (3630 F) in an inert atmosphere environment.
Oxy-Gon developed a system using a commercially available bench-top type, 25-ton (222 kN) post and platen laboratory pres. This type of hydraulic press uses a manually operated cylinder mounted on the bottom platen as opposed to an automated electromechanical hydraulic cylinder normally mounted on a split top platen. The furnace hot zone (Fig. 6) is designed to be compatible with an existing power supply.
The versatile compact furnace design includes:
- Double walled, water-cooled stainless steel chamber.
- Insulation package of two layers, each 0.010 in. (0.25 mm) thick tungsten sheet backed by four layers of 0.010 in. thick molybdenum sheet separated with helical wound strips.
- Tantalum sheet heating element.
- Work-zone dimensions of 3 in. diameter by 4 in. high (about 75 by 100 mm)
- Suitable for mechanical and molecular vacuum pumps.