Method of manufacture for a lightweight, high-precision silicon carbide mirror assembly

An aerospace mirror having a reaction bonded (RB) silicon carbide (SiC) mirror substrate, and a SiC cladding on the RB SiC mirror substrate forming an optical surface on a front side of the aerospace mirror. A method for manufacturing an aerospace mirror comprising obtaining a green mirror preform comprising porous carbon, silicon carbide (SiC), or both, the green mirror preform defining a front side of the aerospace mirror and a back side of the aerospace mirror opposite the front side; removing material from the green mirror preform to form support ribs on the back side; infiltrating the green mirror preform with silicon to create a reaction bonded (RB) SiC mirror substrate from the green mirror preform; forming a mounting interface surface on the back side of the aerospace mirror from the RB SiC mirror substrate, and forming a reflector surface of the RB SiC mirror substrate on the front side of the aerospace mirror. Additionally, the method can comprise cladding the reflector surface of the RB SiC mirror substrate with SiC to form an optical surface of the aerospace mirror.

BACKGROUND

Space-based imaging systems (e.g., in telescopes commonly found on satellites) are often configured as reflector-type imaging systems. Such imaging systems require large primary mirrors (i.e., mirror diameter greater than ˜0.5 meter) with large apertures to achieve a desired resolution and/or signal-to-noise ratio (SNR). Spaced-based imaging systems are susceptible to environmental (e.g., temperature) variations that can reduce image quality. Mirrors used in such imaging systems are therefore designed to reduce sensitivity to environmental variation. In addition, surface distortion of a mirror's optical surface, or “figure error,” is typically held to very tight tolerances (e.g., a fraction of a millionth of an inch). The larger the mirror the more difficult it is to avoid distortion and meet the tolerance requirements. Materials and designs for high-precision, space-based optical applications are characterized by a low coefficient of thermal expansion (CTE), light weight, and high stiffness in order to meet design requirements. One typical mirror design utilizes near-zero CTE material for optics (e.g., ultra-low expansion (ULE) glass mirrors) in the system. Silicon carbide (SiC) is also an emerging material for use in high-precision optics.

DETAILED DESCRIPTION

An initial overview of the inventive concepts are provided below and then specific examples are described in further detail later. This initial summary is intended to aid readers in understanding the examples more quickly, but is not intended to identify key features or essential features of the examples, nor is it intended to limit the scope of the claimed subject matter.

Although large ULE glass mirrors have been effectively utilized in high-precision, space-based optical applications, these mirrors are heavy and difficult to produce. The low stiffness-to-weight ratio of ULE glass material results in self-weight deflection in large ULE glass mirrors (e.g., primary mirrors in reflector telescope systems), which complicates alignment and testing on the ground. SiC, on the other hand has excellent mechanical properties, but cannot be polished to the necessary surface finish. A SiC mirror therefore requires a cladding material, such as amorphous elemental silicon, to provide an acceptable surface finish of the final mirror. In addition, some forms of SiC, such as sintered SiC commonly utilized in SiC mirrors, exhibit shrinkage (e.g., greater than 35%) in the manufacturing process, thus requiring over-processing and limiting the size of the mirror that can be produced. The materials currently used to meet the requirements of high-precision, space-based optical systems are expensive to manufacture and require long lead times to produce. A trade-off between weight, thermal expansion, and stiffness is often required to meet cost and schedule constraints.

Accordingly, an aerospace mirror and associated manufacturing methods are disclosed that provide a light-weight, high-stiffness, and low thermal expansion mirror with excellent optical properties, which can be manufactured at lower cost and on a shorter schedule than with legacy materials. The aerospace mirror can include a reaction bonded (RB) SiC mirror substrate, and a SiC cladding on the RB SiC mirror substrate forming an optical surface on the front side of the aerospace mirror.

A method for manufacturing an aerospace mirror can comprise obtaining a green mirror preform comprising porous carbon, SiC, or both, the green mirror preform defining a front side of the aerospace mirror and a back side of the aerospace mirror opposite the front side. The method can also comprise removing material from the green mirror preform to form support ribs on the back side. The method can further comprise infiltrating the green mirror preform with silicon to create a RB SiC mirror substrate from the green mirror preform. The method can still further comprise forming a mounting interface surface on the back side of the aerospace mirror from the RB SiC mirror substrate, and forming a reflector surface of the RB SiC mirror substrate on the front side of the aerospace mirror. Additionally, the method can comprise cladding the reflector surface of the RB SiC mirror substrate with SiC to form an optical surface of the aerospace mirror.

A method for preparing an optical surface of an aerospace mirror is also disclosed. The method can comprise obtaining an aerospace mirror having an optical surface on a front side and a mounting interface surface on a back side, the mounting interface surface being operable to mount or facilitating the mounting of the aerospace mirror to an external support structure in a final installation by coupling with a final installation mounting structure. The method can also comprise assembling the aerospace mirror to a test support base by coupling a test mounting structure to the mounting interface surface, wherein the test mounting structure is the same as, or equivalent to, or otherwise corresponds to, the final installation mounting structure. The method can further comprise measuring the optical surface (i.e., while attached to the test mounting structure). Additionally, the method can comprise machining the optical surface.

In addition, an aerospace mirror manufacturing assembly is disclosed. The assembly can comprise a test support base. The assembly can also comprise a test mounting structure. Additionally, the assembly can comprise an aerospace mirror mounted to the test support base via the test mounting structure. The aerospace mirror can have an optical surface on a front side and a mounting interface surface on a back side coupled to the test mounting structure. The mounting interface surface can be operable to mount the aerospace mirror to an external support structure in a final installation by coupling with a final installation mounting structure. The test mounting structure can be the same as, or equivalent to, or otherwise correspond to, the final installation mounting structure.

To further describe the present technology, examples are now provided with reference to the figures. With reference toFIGS. 1A and 1B, one embodiment of an aerospace mirror100is schematically illustrated in front and back views showing respective front (FIG. 1A) and back (FIG. 1B) sides of the mirror100, which are opposite one another. Detailed cross-sectional views of the mirror100are shown inFIGS. 2 and 3. In some embodiments, the mirror100can be configured as a primary mirror in a reflective mirror system, which may be used in optical telescopes (e.g., high resolution imaging systems) and infrared sensors.

In general, the mirror100can include a mirror substrate and a reflective or optical surface on the front side of the mirror100disposed on or otherwise supported by the substrate. For example, as illustrated in cross-section inFIG. 2, a mirror substrate110can have a reflector surface111on a front side101aof the mirror100. An optical surface112can be formed on the reflector surface111of the substrate110. The reflector surface111can provide the basic structure for the mirror's optical surface112, which can be formed of a thin layer of material or cladding113disposed on the reflector surface111, as discussed in more detail below. In some embodiments, the reflector surface111can form one side of a facesheet114of the mirror substrate110.

The mirror substrate110can have any suitable configuration. In one aspect, the mirror substrate110can be configured to include weight-reduction structural features, such as support ribs115on the back side101bof the mirror100(e.g., an “open back” configuration), which can provide support for the reflector surface111and optical surface112of the mirror100. For example, as shown inFIG. 2, the ribs115can extend from the facesheet114to provide a structural backing for the reflector surface111and the mirror's optical or reflective surface112. The ribs115can be in any suitable arrangement or configuration to provide adequate structural support for the mirror100. As illustrated inFIG. 1B, the ribs115can be arranged in a triangular isogrid pattern, which can provide a lightweight and stiff substrate structure for the mirror's optical or reflective surface112. However, other rib patterns may be utilized, and are contemplated herein.

The mirror100can also include one or more mounting interface surfaces (e.g., bond pads) on the back side101bof the mirror100to mount the mirror100to an external support structure for the mirror100, such as a satellite (e.g., via a mounting structure, such as a strut (not shown)). In one aspect, mounting interface surfaces can be formed in or on the mirror substrate110. In the embodiment shown inFIG. 3, a mounting interface surface116(e.g., a bond pad) can be formed on a rib115of the substrate110, for example, to facilitate bonding with an attachment fitting. Examples of such mounting interface surface and rib configurations are disclosed in United States Patent Application Publication No. 2017/0055731, which is incorporated herein by reference in its entirety. The rib115can have any suitable profile117a-c, which may be configured to facilitate coupling with an attachment fitting (i.e., a mounting structure) and/or to reduce deformation due to the coupling with the attachment fitting. In other embodiments, a mounting interface surface116can define a cylindrical hole (not shown) at an intersection of the support ribs115on the back side101bof the mirror. The cylindrical hole can be used to bond or screw a metal insert or fitting to the mirror100for attachment to an external support structure.

In one aspect, the mirror substrate110can comprise a reaction bonded (RB) silicon carbide (SiC) material. In another aspect, the optical surface112is formed from a SiC cladding113on the RB SiC mirror substrate110. In particular, the SiC cladding113can comprise a chemical vapor deposition (CVD) SiC material. In some embodiments, the mirror100can include an optical coating118(e.g., a high-reflectivity, thin film coating) on the optical surface112, as shown inFIG. 2.

A method202is outlined inFIG. 4for manufacturing an aerospace mirror, such as the mirror100constructed of an RB SiC substrate110and a SiC cladding113for the optical surface112. In general, the method comprises obtaining a green mirror preform220, removing material from the green mirror preform221, infiltrating the green mirror preform with silicon to create an RB SiC mirror substrate from the green mirror preform222, selectively removing material to form precision features in the RB SiC mirror substrate223, and cladding the RB SiC mirror substrate with SiC to form an optical surface224.

The green mirror preform obtained at step220can comprise any material or combination of materials (e.g., porous carbon, SiC, plasticizers, fillers, etc.) suitable for reaction bonding by infiltration with silicon to form RB SiC material (e.g., comprising a SiC fraction of 70% and a Si filling of 30%, although other relative compositions are contemplated). The green mirror preform can be fabricated utilizing any suitable technique or process known in the art (e.g., made of a dried SiC and carbon slurry). RB SiC is a high specific stiffness, low thermal expansion material with excellent mechanical properties for an aerospace mirror. The green mirror preform can have any suitable shape or configuration and can be made or constructed by any suitable process known in the art. Typically, the green mirror preform will have a shape that corresponds to an outer boundary shape of the final mirror substrate. Thus, the green mirror preform can define the general shape of the mirror as well as the front and back sides of the mirror. The green mirror preform may be manufactured to rough tolerances and sized to exceed the final size of the mirror substrate sufficient to allow for material removal in the green state, shrinkage due to silicon infiltration and formation of RB SiC, and selective material removal in the “hardened” RB SiC state to form precision features. One advantage of RB SiC material is low shrinkage from the green state when compared to other types of SIC, such as sintered SiC.

Material can be removed from the green mirror preform at step221to form various features of the mirror substrate110. For example, material can be removed from the green mirror preform to form support ribs115on the back side101bof the mirror100(e.g., in an isogrid or other pattern) and to form a back side of the facesheet114, which can reduce weight and areal density of the mirror100. In some embodiments, the material can be removed from the front side101aof the green mirror preform to form a reflector surface111of the mirror substrate110. In other embodiments, the green mirror preform can be provided with a surface corresponding to the reflector surface111of the substrate110that can be formed into RB SiC without prior material removal in the green state. Material can be removed from the green mirror preform utilizing any suitable technique or process. One benefit of the green state material for RB SiC is the speed and ease in which it can be machined. Thus, typically, material will be removed from the green mirror preform by machining (e.g., milling). In some embodiments, high speed machining can be utilized. The ability to rapidly machine the material in the green state can significantly reduce the cost and schedule of producing a large, lightweight mirror blank from SiC by other methods, such as by casting to final dimensions.

Once material has been removed from the green mirror preform at step221, the green mirror preform can be infiltrated222with silicon to create an RB SiC mirror substrate110from the green mirror preform. Reaction bonded silicon carbide is made by a chemical reaction between carbon or graphite with gaseous and/or molten silicon. Silicon infiltration to form reaction bonded SiC can be accomplished by any suitable technique or process known in the art, such as infiltration with silicon in a gaseous and/or molten state. The silicon reacts with the carbon to form silicon carbide (additional SiC if some was present in the preform). The reaction product bonds the silicon carbide particles. Any excess silicon fills the remaining pores in the body and produces a dense SiC—Si composite. Due to the left-over traces of silicon, reaction bonded silicon carbide is often referred to as siliconized silicon carbide.

With a mirror substrate110made of RB SiC, material can be selectively removed to form at step223small precision features and/or critical features in the mirror substrate110that meet dimensional, form, and/or surface finish requirements. For example, one or more mounting interface surfaces116(e.g., small bond pads) can be formed on the back side101bof the mirror100from the RB SiC mirror substrate110. In addition, the reflector surface111of the RB SiC mirror substrate110can be formed on the front side101aof the mirror100, which may be a precision and/or critical feature due to its impact on optical image quality. Material can be removed from the RB SiC mirror substrate110utilizing any suitable technique or process, such as machining (e.g., milling, lapping, grinding, etc.). Because RB SiC is a hard material that is difficult and time-consuming to machine, machining of the hardened SiC can be limited to only high-precision features (e.g., mounting interfaces and optical-related surfaces) that may not be accurately formed following green state machining and subsequent deformation (e.g., shrinkage, although minimal) due to reaction bonding of the silicon carbide. The amount of material removed during such fine machining of the hard SiC material can be minimized by the design of the green mirror preform and/or the location or amount of material removed from the green mirror preform prior to forming RB SiC. The strategic removal of material in the green state and in the hardened SiC state can provide a lightweight mirror with a low areal density while minimizing high-precision machining time on hard SiC.

Once the reflector surface of the RB SiC mirror substrate is in an acceptable condition, the mirror substrate can be clad at step224with SiC to form the optical surface112of the mirror100. In some embodiments, the SiC cladding113can be applied or formed by a chemical vapor deposition (CVD) process (e.g., about 0.005 inches thick), a process in which volatile compounds containing carbon and silicon are reacted at high temperatures in the presence of hydrogen. Cladding the RB SiC mirror substrate110with CVD SiC can facilitate forming the optical surface112because CVD SiC material is easier to polish than RB SiC material. CVD SiC has a coefficient of thermal expansion close to that of RB SiC and, when applied only to the reflector surface111of the mirror substrate110, can allow the optical form to be held over the operational temperature range. Compared to amorphous, elemental silicon as a cladding for optical surfaces, which is the industry standard, CVD SiC provides better surface quality and can be polished in less time. The combination of the RB SiC mirror substrate110and the CVD SiC cladding113for the optical surface112can provide a relatively low-cost, high-quality mirror.

In some embodiments, the SIC optical surface112can be subjected to grinding (e.g., loose abrasive grinding) and/or polishing (rough and/or fine polishing) operations to achieve a desired surface quality. An optical coating (e.g., a high-reflectivity, thin film coating) can then be applied or coated on the optical surface112as typical and known in the art for precision optics.

A method303for preparing an optical surface of an aerospace mirror (e.g., to achieve final dimensions, form, and/or surface finish) is outlined inFIG. 5. In general, the mirror100(e.g., RB SiC substrate110with CVD SiC cladding113as disclosed herein) has an optical surface112and one or more mounting interface surfaces116operable to mount the mirror100to an external support structure (e.g., a metering structure of a satellite) in a final installation by coupling with a final installation mounting structure (e.g., flight mounts or mounting struts). The method303can comprise assembling at step330the aerospace mirror100to a test support base using a test mounting structure that is the same as, or equivalent to, or that otherwise corresponds to the final installation mounting structure, measuring the optical surface112(e.g., for form error, surface roughness, surface quality, etc.) at step331, and machining the optical surface112(e.g., milling, grinding, and/or polishing) at step332. Measuring331the optical surface112and machining332the optical surface112can be repeated until the optical surface112is within a given tolerance.

Aspects of an aerospace mirror manufacturing assembly404that can be used to prepare an optical surface412of an aerospace mirror400are illustrated inFIGS. 6-9. In particular,FIGS. 6-8show schematic representations of front, side, and rear views, respectively, of the mirror manufacturing assembly404, andFIG. 9shows a detail view of the mirror400coupled to a mounting structure.

In addition to the aerospace mirror400, the assembly404can include a test support base440and a test mounting structure441(e.g., mounting struts). The mirror400can be mounted to the test support base440via the test mounting structure441prior to final polish of the optical surface412. The test mounting structure441is the same as, or equivalent to, or otherwise corresponds to the final installation mounting structure. In other words, the mirror400can be mounted on a test mounting structure441that mimics or is similar to the final mounting configuration so that the mirror400experiences the same or similar type and/or amount of deformation during the final stages of manufacture that it will experience in its final mounting configuration for use. Typically, mounting a mirror in its final (i.e., flight) mounting configuration (e.g., on a satellite) will induce significant wavefront error due to deformation of the mirror's optical surface, which may be due to mounting hardware misalignment and/or bonding errors resulting from bonding mounting hardware to the mirror. Mounting the mirror in a set-up that is the same or similar to the final mounting configuration causes this deformation to occur during manufacturing (e.g., prior to final polishing). As a result, the final material removal processes (e.g., final polishing) can account for the assembled deformation. In other words, the mirror can be in its final as-assembled configuration during the final stages of manufacture so that any deformation introduced in this configuration can be removed by subsequent material removal processes. Accordingly, the final mirror produced is within tolerance when actually mounted in its final (i.e., flight) configuration, such as on a satellite. This approach can therefore introduce fewer errors in the overall assembly error stack-up and prevent mounting and bonding error from being evident in the final optical surface.

The test mounting structure441can have any suitable configuration. In some embodiments, the test mounting structure441can comprise one or more struts442a-f. Examples of such struts are disclosed in United States Patent Application Publication No. 2017/0055731 referred to above. In the test mounting structure441configuration, the number of struts442, the angle/orientation of the struts442relative to the mirror400(e.g., to ribs415), the length of the struts442, etc. can be the same or configured to produce an equivalent effect on the mirror400as the final mounting structure. In some embodiments, one or more struts442can have an adjustable length along an axis445(FIG. 9) to facilitate adjusting a position of the mirror400. In some embodiments, one or more struts442can have a negative coefficient of thermal expansion (GTE). Examples of such struts are disclosed in U.S. patent application Ser. No. 15/828,223 filed Nov. 30, 2017 and titled “Multi-Material Mirror System,” which is incorporated herein by reference in its entirety.

In a particular embodiment, the struts442a-fcan be coupled to support ribs415of the mirror400, as shown inFIG. 7. As illustrated, the struts442a-fcan be aligned with the ribs415to which they are attached to efficiently transfer loads between the mirror400and the test support base440. In this case, six struts442a-fare arranged in three sets or pairs (e.g., bi-pods), which are aligned with the ribs415or rib planes443a-coriented in three directions. In addition to the struts442a-f, the test mounting structure441can include attachment fittings444(e.g., a clevis, seeFIG. 9). The attachment fitting444can be bonded (e.g., with structural adhesive) or otherwise attached directly to the ribs415to which the struts442a-fare aligned. Thus, the mirror400can have a mounting interface surface416(e.g., a bond pad) coupled to the test mounting structure441.

With the struts442a-fin-line with the ribs415or rib planes443a-c, out-of-plane loads can be minimized or prevented from being imparted onto the ribs415. Such a configuration can achieve an extremely efficient load transfer path from the strut mounts into the mirror400and can enable the mirror's optical form to be minimally affected by operational temperature changes, particularly when low CTE materials are utilized in the mounting structure (e.g., Invar® attachment fittings444). This strut configuration can be identical or kinematically equivalent to a final mounting configuration for the mirror when installed, for example, on a satellite. The stiff RB SiC material of the mirror400and its lightweight design can facilitate the use of lightweight kinematic mounts, such as the struts442a-f, which can minimize thermal sensitivity and self-weight deflection of the mirror400.

In one aspect, the hardware used to mount the mirror400to the test mounting structure441can be the same hardware used to mount the mirror400in its final mounting configuration when installed. For example, the struts442a-fused to mount the mirror400in the assembly404can also be used to mount the mirror400in its final flight mount configuration. In addition, the attachment fittings444, which may be bonded to the mirror400, can remain permanently attached to the mirror400even if different struts may be used in the final flight mount hardware.

With the mirror400mounted as in, or equivalent to, the final flight mount configuration in the assembly404, the optical surface412can be measured by optical testing of the optical surface412(e.g., optical metrology to ascertain form error). Any suitable optical test or metrology technique can be utilized. In some embodiments, a rotational shearing optical metrology technique can be employed. Rotational shearing can provide high accuracy in determining the mirror's surface figure while reducing the influence of gravity in the flight mounting configuration, while also allowing use of a low-cost horizontal optical test. Thus, the optical metrology for the mirror400can be done using rotational shearing techniques to remove the influence of gravity while the mirror400is mounted in the assembly404.

The optical surface412can be fine or finish machined while the mirror400is in the assembly404or, in other words, after mounting hardware has been attached to the mirror400(e.g., attachment fittings444have been bonded to the bond pads416and coupled to structural supports, such as the struts442a-f) and the mirror400has been mounted to the test support base440. The optical surface412can be finish machined utilizing any suitable process or technique, such as polishing, grinding, lapping, etc. In one aspect, the assembled mirror400can be polished using rotational shearing techniques to remove the influence of gravity while mounted on the structural supports. This can account for any mechanical influence of the structural supports on the optical form. In other words, any mechanical influence of the mounting structure441on the optical form can be accounted for in the manufacturing process and removed during final machining processes (e.g., polishing). The use of flight or flight-like mounts during manufacturing and optical testing can therefore remove at least one factor from the potential optical form error sources.

In one aspect, the CVD SiC cladding material can facilitate polishing the optical surface. For example, the CVD SiC cladding that forms the optical surface412is easier to polish than the RB SiC material that forms the mirror substrate110ofFIGS. 1A, 1B, 2, and 3. The CVD SiC cladding113can enable the optical surface412to be polished to better than 5 angstroms of RMS surface roughness, with excellent optical form and control of the optical surface412.

Once the optical surface412is in an acceptable condition, the optical surface412can then be coated with a suitable optical coating (e.g., a high-reflectivity thin film coating) as known in the art.

Although the disclosure may not expressly disclose that some embodiments or features described herein may be combined with other embodiments or features described herein, this disclosure should be read to describe any such combinations that would be practicable by one of ordinary skill in the art. The user of “or” in this disclosure should be understood to mean non-exclusive or, i.e., “and/or,” unless otherwise indicated herein.