Method of making whisker reinforced high fracture toughness ceramic threaded fasteners

A high temperature fastener including a bolt and a nut, where the bolt and the nut are constructed of an aluminum oxide ceramic material reinforced with silicon-carbide crystal whiskers or silicon nitride.

FIELD

This disclosure pertains to a fastener comprised of a bolt and nut where the bolt and nut are constructed of an aluminum oxide ceramic material reinforced with silicon-carbide crystal whiskers.

BACKGROUND

Thermal protection systems (TPS), for example re-entry heat shields for spacecraft, fuselage sections of hypersonic vehicles, jet engine exhaust components, etc. are constructed of materials that need to be heat-resistant and must endure very harsh environments. Reentry vehicle surfaces are particularly difficult. The surface must have low catalycity because the shockwave just in front of the reentry vehicle surface dissociates the air molecules and provides the potential for additional heating. As the air molecules break apart and collided with the surface they recombine in an exothermic reaction. Since the surface acts as a catalyst, it is important that the surface has a low catalycity, this will reduce the propensity to augment the energy from this chemical reaction. These materials must also be resistant to hot oxygen, particularly resistant to atomic oxygen to minimalize scaling of the material surfaces. The materials must have high emissivity to ensure the maximum rejection of incoming convective heat through radiative heat transfer. These requirements are difficult to meet in thermal protection system applications such as tiles, blankets and other similar structures used in the thermal protection systems.

In thermal protection systems that employ tiles, blankets and ceramic matrix composite components, the tiles for example are primarily bonded in place. For many TPS applications, adhesively bonding insulation such as tiles is used to attach insulation to the outer mold lines of vehicles, for example hypersonic vehicles. There is an interest in mechanically attaching tiles, blankets and other forms of ceramic matrix composites for easy, quick replacements, or for maintenance, as well as the limitation in temperature of many adhesives.

However, in applications such as heat shield surfaces of re-entry vehicles, engine exhaust components and in hypersonic vehicle constructions, the use of metal fasteners in attaching ceramic matrix components in these applications has been a problem. Most metals have high catalycity, low thermal emissivity, a high coefficient of thermal expansion and get soft and weaker with increases in temperature. If fasteners are used to attach TPS or exhaust liners to a vehicle, they are usually made of high temperature metal alloys. TPS components or panels if fastened in place, the fasteners are usually buried or covered with an insulated plug to protect the fastener and assure a surface that is low in catalycity and high emissivity and smooth so as not to disturb or trip the boundary layer which would raise the temperature. This protects the metal fastener from the extreme heat of its operational environment, but there are disadvantages in not being able to get to a metal fastener buried in the TPS very easily, or at all. Additionally, most metals have high catalycity, low emissivity, a high coefficient of thermal expansion and do not meet many of the requirements for fasteners used in high temperature environments, no less have a low coefficient of thermal expansion to match ceramic matrix composites. Unlike metal, most ceramics have low catalycity, some have high emissivity and are very hard and wear resistant.

Presently, most turbine engine exhaust components and fasteners are mainly if not all made out of super alloy metals. The components are actively cooled so that the metal can survive the environment. As ceramic matrix composites and other ceramic components get implemented into turbine engine exhaust systems, the metal super alloy fasteners can no longer be used because the metal cannot take the temperature. This is made worse by the ceramic matrix composite having a lower thermal conductivity compared to metal, so even if the panels were cooled, the fasteners would still have a tendency to overheat.

SUMMARY

For the above set forth reasons and others, it would be much better for a fastener used to fasten ceramic matrix composites in a thermal protection system to be constructed of a ceramic material. However, most strong ceramics are monolithic, brittle, notch sensitive, have thermal shock issues and are prone to catastrophic failure, which is not ideal for making fasteners. Because ceramic fasteners are brittle, hard and notch sensitive, machining screw threads on a fastener of ceramic material is also very difficult, especially machining internal screw threads. Creating threaded ceramic fasteners is usually done in processes like injection molding before firing, but these types of threads are rounded and not precise due to firing shrinkage, and the ceramic fastener strength is still typically very low, with high scatter, and are not very predictable.

The high temperature threaded fastener of this disclosure is comprised of a bolt having an external screw threaded surface and a nut having an internal screw threaded surface. Both the bolt and nut are constructed of an aluminum oxide ceramic material reinforced with silicon-carbide crystal whiskers. The high temperature threaded fastener meets the requirement of high strength over an entire temperature range in which the fastener will be exposed, with high fracture toughness, minimal notch sensitivity, low catalycity, high thermal emissivity, high stiffness, high hardness, good thermal shock resistance and not scaling by hot atomic oxygen. Ceramics including alumina are naturally low in catalycity, the opposite of most metals. The silicon-carbide crystal whiskers with aluminum oxide not only improve fracture toughness, but also increase the fastener's emissivity. Again the opposite of metal which have very low emissivity. The aluminum oxide ceramic material reinforced with the silicon-carbide crystal whiskers also has a coefficient of thermal expansion that closely matches the coefficient of thermal expansion of oxide ceramic matrix components with which the fastener is used.

In constructing the fastener bolt, a mixture of the aluminum oxide ceramic material powder and the silicon-carbide crystal whiskers is prepared. The mixture is then hot pressed at a high temperature and high pressure, forming a blank having a head and a shaft.

The aluminum oxide ceramic material and reinforcing silicon-carbide crystal whiskers produce a blank of hard ceramic material with high fracture toughness that can be machined. A tool interface surface, for example a hex head is machined on the head of the blank. A screw thread exterior surface is machined on the shaft of the blank. In the valley of the threads, the threads are not cut to ASTM standards, but instead are cut so that the valleys are shallow, reducing chance of crack initiation and notch sensitivity.

In constructing the fastener nut, in a similar manner to that of the fastener bolt, a mixture of the aluminum oxide ceramic material powder reinforced with the silicon-carbide crystal whiskers is prepared. The mixture is then hot pressed. To form the internal screw threads in the nut, a graphite pre-form is machined with external screw threads that are complementary to the internal screw threads of the nut. The pre-form is placed inside the powder mixture so that during compaction and heating of the mixture into a dense finished ceramic blank, the internal threads of the nut are formed around the graphite pre-form. After the hot pressing of the mixture forming the blank is completed, the much softer graphite pre-form is cleaned out of the blank leaving internal female screw threads in the fastener nut to exact dimensions since the threads were formed under pressure during sintering, eliminating shrinkage normally associated with firing ceramics. The outside configuration of the fastener nut can then be machined to the desired configuration.

Unlike TPS applications, turbine engine components are run for long periods of time and are usually actively cooled, to allow material to survive the harsh environment. While ceramic fasteners can take higher temperatures than metals in some cases, the ceramic fastener needs to be cooled. Especially since the thermal conductivity of most ceramics and ceramic matrix composite components have fairly poor thermal conductivity compared to metals. In a similar manner to constructing the nut, pre-forms can be used in constructing the fastener bolt with internal cooling channels, internal screw threading and/or internal tool interface surfaces such as an allen wrench or a Torx interface surface that would be very expensive if not impossible to machine.

DESCRIPTION

FIG. 1is a representation of a perspective view of the fastener10of this disclosure which is comprised of a bolt12or the bolt and a nut14.FIG. 2is a side elevation view of the bolt12and nut14. InFIG. 2, the opposite side of the bolt12and nut14is a mirror image of the side view of the bolt12and nut14represented inFIG. 2. As represented inFIGS. 1 and 2, the configurations of the bolt12and nut14are conventional.

The bolt12is shown with a head16and a shaft18with a screw threaded exterior surface22. The bolt head16is shown having a hex-shaped tool interface surface24. The hex-shaped configuration of the bolt head tool interface surface24is only one example of the tool interface surface configuration the bolt head16could have. The bolt head16could be constructed with any other equivalent tool interface surface configuration.

The fastener nut14has a cylindrical interior bore26with a screw threaded interior surface28surrounding the interior bore26, The screw threaded interior surface28is formed complementary to the screw threaded exterior surface22of the bolt12, enabling the nut14to be screw threaded on the bolt12. The nut14is also formed with a hex-shaped tool interface surface32on the exterior of the nut. As with the fastener bolt12, the fastener nut14could be constructed with any other equivalent tool interface surface configuration.

As stated earlier, the configurations of the fastener bolt12and the fastener nut14are conventional. What makes the fastener bolt12and fastener nut14unique is that they are constructed as a high temperature threaded fastener. This is achieved by both the bolt12and nut14being constructed of a ceramic composite that uses the technology of whisker reinforcement. The hard ceramic matrix is reinforced with extremely strong, stiff, silicon-carbide crystals, commonly called whiskers. Both the bolt12and nut14are constructed of a ceramic matrix composite material that is a mixture of aluminum oxide ceramic material reinforced with silicon carbide crystal whiskers. One example of a ceramic matrix composite material used to construct the bolt12and nut14is the whisker reinforced ceramic material WG-300®, which is a registered trademark of Greenleaf Corporation. In WG-300®, the percentage of silicon-carbide crystal whiskers in the mixture of aluminum oxide ceramic material powder and the silicone carbine crystal whiskers is approximately 30%. In other examples of the ceramic composite material used to construct the bolt12and nut14, the percentage of silicon-carbide crystal whiskers in the mixture of aluminum oxide ceramic material powder and the silicon-carbide crystal whiskers is in a range of 10%-30% of the mixture.

The method of constructing the high temperature threaded fastener bolt12is represented inFIG. 3. In the construction of the bolt12, a mixture34of the aluminum oxide ceramic material powder and the silicone-carbide crystal whiskers is prepared. The mixture34of the aluminum oxide ceramic material powder and the silicon-carbide crystal whiskers is put into a high temperature press36for forming a blank to be used in constructing the fastener bolt12.FIG. 3shows a representation of a high temperature high pressure press36. InFIG. 3the aluminum oxide ceramic material powder38and the silicon-carbide crystal whiskers42are represented schematically and are not shown to scale. The press36has mold die pieces44,46that are configured to form a blank for the bolt12from the aluminum oxide ceramic material powder and the silicon-carbide crystal whiskers of the mixture34. The mixture34is positioned in the press36between the press dies44,46and is hot pressed at a high temperature (over 3,000 degrees Farenheit) and compressed at a high pressure, forming a blank48of the bolt12having a head52and a shaft54as represented inFIG. 4. The blank48is dense and has a fine grain size. The external pressure applied to the mixture34simultaneously with the temperature of the press36produces good consolidation of the aluminum oxide ceramic material and the reinforcing silicone-carbide crystal whiskers.

The aluminum oxide ceramic material and the reinforcing silicon-carbide crystal whiskers54produce the blank48of hard ceramic material with high fracture toughness that can be machined.

A tool interface surface, for example the hex-shaped bolt head24represented inFIGS. 1 and 2is then machined on the head52of the blank48. Alternatively, the tool interface surface24could be molded on the blank head52. A screw threaded exterior surface22, for example that represented inFIGS. 1 and 2is machined on the shaft54of the blank48. The screw threaded exterior surface22is machined on the shaft54of the blank48with the bottoms of the valleys of the screw threads being made shallow to further reduce the notch sensitivity of the fastener10under load. The whiskers42in the blank48help keep microcracks from forming and propagating through the shaft54of the blank48during machining of the blank. The whiskers42in the bolt12also make the screw threads22machined on the bolt12much less notch sensitive. The whiskers42also help keep microcracks from forming and propagating through the fastener10under load in use of the fastener.

The method of constructing the fastener nut14is similar to that of the fastener bolt12and is represented inFIG. 5. In the construction of the nut14, the mixture34of the aluminum oxide ceramic material powder38and the silicon-carbide crystal whiskers42is prepared. The mixture34of the aluminum oxide ceramic material powder38and the silicon-carbide crystal whiskers42is put into a high temperature press56for forming a blank to be used in constructing the fastener nut14.FIG. 5shows a representation of a high temperature high-pressure press56. InFIG. 5the aluminum oxide ceramic material powder38and the silicon-carbide crystal whiskers42are represented schematically and are not shown to scale. The press56and has mold die pieces58,62that are configured to form a blank for the nut14from the aluminum oxide ceramic material powder and the silicon-carbide crystal whiskers of the mixture34. The mixture34is positioned in the press56between the press die pieces58,62and is hot pressed at a temperature of over 3000° Fahrenheit while the mixture is compressed at a high pressure to form a blank of the nut14. The blank64of the nut14is dense and has a fine grain size. External pressure applied to the mixture34simultaneously with the temperature of the press56produces a good consolidation of the aluminum oxide ceramic material and the reinforcing silicon-carbide crystal whiskers. The aluminum oxide ceramic material and the reinforcing silicon-carbide crystal whiskers produce the blank64of the nut14of hard ceramic material with high fracture toughness.

In developing the method of forming internal screw threads in the blank64of the nut14, it was recognized that it would be very difficult, if possible to machine internal screw threads in the very hard ceramic material of the nut blank64, at least cost efficiently. To form the screw threaded interior surface28in the nut14, a graphite preform66is machined with external screw threads68that are complementary to the screw threaded interior surface28of the nut14. As represented inFIG. 5, the preform66is placed inside the mixture34in the press56. During heating and compression of the mixture34in the press56into the dense, finished ceramic blank64of the nut14, the screw threaded interior surface28of the nut14is formed around the graphite preform66. After the hot pressing of the mixture34forming the nut blank64is completed, the soft graphite preform66having the external screw thread68is easily cleaned out of the nut blank64, leaving a cost-efficient, clean, precise screw threaded interior surface28in the nut blank64. Because the screw threaded interior surface28is formed during the pressure sintering around the preform66, no shrinkage of the screw threaded interior surface28occurs. This enables the production of a high tolerance screw threaded interior surface28that matches closely to the machined screw threaded exterior surface22on the mating fastener bolt12.

As an alternative to using the mixture of aluminum oxide ceramic material powder and silicon-carbide whiskers in constructing the bolt12and nut14, silicon nitride (Si3N4) could be used in their place.

A tool interface surface, for example the hex shaped exterior surface32of the nut14is then machined on the nut14. Alternatively, the tool interface surface32could be molded on the nut blank64.

FIG. 6is a representation of a method of constructing the high temperature threaded fastener bolt72with cooling channels and/or internal tool interface surfaces. In the method of construction represented inFIG. 6, the fastener bolt72is constructed in the same manner as the fastener bolt12described earlier. However, when the mixture34of the aluminum oxide ceramic material powder and silicon-carbide crystal whiskers is put into the high temperature press36, one or more graphite preforms74,76are positioned in the mixture34.

One of the preforms74represented inFIG. 6is positioned at the center of the mixture34in press36along the length of the bolt72to be formed. This preform74could have a cylindrical exterior configuration to form a cylindrical cooling channel through the center of the fastener bolt72. Alternatively, or in addition to forming a cooling channel, the preform74at the center of the mixture34could have a configuration to form an internal tooling surface in the fastener bolt72, for example a hexagon cross-section to form a hexagonal internal tool interface surface in the fastener bolt72. The preform74with the hexagon cross-section would form an internal tool interface surface having a hexagonal configuration that could be engaged by an allen wrench.

In addition to the preform74are positioned in the mixture34along the length of the bolt12to be formed, a further preform76could be positioned in the mixture34across the width of the bolt72to be formed. As represented inFIG. 6, the additional preform76is positioned in the portion of the mixture34that will be formed as the head of the fastener bolt72. This additional preform76could have a cylindrical exterior surface configuration to form a cylindrical cooling channel that passes through the center of the head of the fastener bolt72.

The press36is operated to heat and compress the mixture34in the press to form the fastener bolt72in the same manner as described earlier. Once the blank for the fastener bolt72is formed and removed from the press36, the graphite preforms74,76can be removed from the blank in the same manner discussed earlier with regard to the fastener nut14. This produces the fastener bolt72with internal cooling channels in the fastener bolt and/or an interior tool interface surface in the fastener bolt.

Fastener bolts and fastener nuts made out of Greenleaf WG-300 with 30% SiC whisker reinforced alumina has a coefficient of thermal expansion (CTE) of 6.0×10−6and WG-150 with 18% SiC whisker reinforcement has a CTE of 7.0×10−6. These fastener has a close CTE match to the oxide CMC using Nextel-720 fiber reinforcement which has an in-plane CTE is 6.56×10−6/C or the oxide CMC with the Nextel-610 fiber has an in-plane CTE of 7.87×10−6/C.

As an alternative to using the mixture of aluminum oxide ceramic material powder and silicon-carbide crystal whiskers in construction of the bolt and nut fasteners, silicon nitride (Si3N4) could be used in their place. While SiC whisker reinforced alumina has a preferred higher fracture toughness of 10 MPa√m compared to 6-7 MPa√m for Si3N4components the CTE is much lower 3.0-3.8×10−6/C which it would make it ideal to be used with non-oxide CMC like C/SiC and SiC/SiC which also has a low CTE in the range of 3.0-5×10−6/C. While the lower fracture toughness of Si3N4 makes it more challenging to fabricate these shapes it still should work for a number of applications.

As various modifications could be made in the construction of the apparatus and its method of operation herein described and illustrated without departing from the scope of the invention, it is intended that all matter contained in the foregoing description or shown in the accompanying drawings shall be interpreted as illustrative rather than limiting. Thus, the breadth and scope of the present disclosure should not be limited by any of the above described exemplary embodiments, but should be defined only in accordance with the following claims appended hereto and their equivalents.