Ultrasonic-Assisted Deep Hole Drilling of Carbon-Carbon Composites for Solid Rocket Motor Nozzle Throat Liners and Combustion Chambers

A manufacturer of aircraft engine nozzle components (C/C composite, requiring 6 threaded holes of 6 mm x 25 mm deep) using conventional carbide drilling had a drill breakage rate exceeding 90 percent and delamination at entry/exit edges (factor > 1.3). Switching to ultrasonic vibration-assisted drilling (40 kHz, diamond-coated carbide, 12 micron amplitude, Vc = 15 m/min, f = 0.005 mm/rev, vortex cold air at -20°C) reduced cutting temperature below 120°C, reduced axial cutting force by 65 percent, eliminated delamination, and increased drill life from less than 1 hole to 200+ holes. Pass rate increased to 98.6 percent (from < 10 percent).

Conventional vs Ultrasonic-Assisted Drilling Methods ​

The drilling of carbon-carbon (C/C) and ceramic matrix composites (CMCs) for aerospace applications presents extreme challenges that conventional twist drilling cannot overcome. Three primary drilling methods are available, each with distinct capabilities and limitations when applied to these abrasive, brittle, and anisotropic materials. The following table provides a comprehensive comparison.

Conventional carbide drilling fails on C/C composites for three fundamental reasons. First, the carbon fibres (2,000+ HV hardness) rapidly abrade the tungsten carbide matrix, causing flank wear exceeding 0.3 mm within 1-5 mm of cumulative cutting. Once the flank wear reaches this level, the cutting forces increase dramatically, and the drill snaps when encountering a fibre bundle oriented perpendicular to the cutting direction. Second, the continuous cutting action generates high temperatures (300-600 C at the hole exit), which causes the carbon matrix to oxidise and the fibre-matrix bond to degrade, resulting in thermal damage that extends 0.5-2.0 mm from the hole edge. Third, the high axial cutting force (200-500 N) causes delamination at the hole entry (where the drill pushes the fibres downward, separating them from the matrix) and at the hole exit (where the drill breaks through the bottom surface, pushing out the last plies). The delamination factor (defined as the ratio of the maximum delamination diameter to the hole diameter) typically exceeds 1.3, which is unacceptable for aerospace components where the holes are subjected to high-temperature gas pressure during operation. Ultrasonic vibration-assisted drilling (UVAD) overcomes all three limitations by superimposing a high-frequency (20-40 kHz), low-amplitude (8-20 micron) axial vibration on the rotating drill. The vibration creates a hammering action that fractures the brittle carbon fibres ahead of the cutting edge, reducing the cutting force by 50-70 percent. The interrupted cutting action (the drill is in contact with the workpiece only during a portion of each vibration cycle) allows coolant to reach the cutting zone between contact cycles, maintaining the temperature below 120 C. Laser drilling eliminates tool wear and cutting forces entirely but produces a heat-affected zone (HAZ) with residual thermal stress and may leave a recast layer on the hole wall that reduces the fibre-matrix bond strength.

Tool Material and Geometry Comparison ​

The selection of tool material and geometry is the most critical decision for successful C/C composite drilling. The tool must withstand extreme abrasion from the carbon fibres while maintaining a sharp cutting edge and adequate chip evacuation. The following table compares the performance of the most commonly used tool materials for C/C and Cf/SiC composite drilling.

CVD diamond-coated carbide drills offer the best combination of wear resistance, edge sharpness, and cost for production drilling of C/C composites. The chemical vapour deposition (CVD) process deposits a layer of polycrystalline diamond (10-20 microns thick) onto a tungsten carbide substrate. The diamond coating provides hardness of 6,000-8,000 HV, which is sufficient to resist abrasion by the carbon fibres for 200-500 holes per tool. The tungsten carbide substrate provides the fracture toughness that prevents catastrophic tool failure when the drill encounters fibre bundles or hard inclusions. The CVD diamond coating thickness must be carefully controlled: a coating that is too thin (< 10 microns) wears through quickly, exposing the carbide substrate to rapid abrasion; a coating that is too thick (> 25 microns) has poor adhesion and may delaminate from the substrate during drilling. The drill geometry for C/C composite drilling differs significantly from metal drilling geometry. The point angle is smaller (90-120 degrees compared to 130-140 degrees for steel) to reduce the axial force component and to minimise delamination at hole entry and exit. The clearance angle is larger (8-12 degrees compared to 5-8 degrees for metal) to reduce friction between the drill flank and the abrasive bore wall. The helix angle is 30-35 degrees, which is a compromise between chip evacuation (higher helix angle is better) and drill stiffness (lower helix angle is stiffer). The drill margin width is reduced to 0.1-0.2 mm (compared to 0.3-0.8 mm for metal drills) to minimise friction with the bore wall. PCD-tipped drills are preferred for micro-deep hole drilling (diameters below 1 mm) because the PCD tip can be ground to a sharper edge than a diamond coating, and the PCD provides better wear resistance for the extreme length-to-diameter ratios (up to 77:1 for 0.2 mm x 15.5 mm holes).

Quality Requirements for Rocket Nozzle Holes ​

The holes in solid rocket motor nozzle throat liners and combustion chambers are critical features that must maintain structural integrity under extreme operating conditions: combustion gas temperatures exceeding 3,000 C, pressures of 50-200 bar, and gas velocities of Mach 2-3. Any defect in a hole can cause a failure that destroys the nozzle and potentially the entire rocket motor. The following table defines the quality requirements, inspection methods, and defect prevention strategies for C/C composite holes in rocket nozzle applications.

The most critical defects are delamination and thermal damage because they compromise the structural integrity of the nozzle around the attachment holes. Delamination creates a separation between the fibre plies that can propagate under the high-pressure gas during rocket motor firing, potentially causing the entire nozzle throat liner to separate from the housing. Thermal damage (matrix oxidation) reduces the fibre-matrix bond strength, allowing the fibres to be eroded by the combustion gas. The inspection of C/C composite holes for rocket nozzles includes 100 percent borescope inspection (visual inspection of each hole for delamination, fibre pullout, and surface defects), 100 percent ultrasonic C-scan of the area around each hole (to detect sub-surface delamination that is not visible on the surface), and 100 percent dimensional inspection (hole diameter, position, verticality, and thread quality). The quality requirements are specified by the propulsion system manufacturer (NASA, ESA, ISRO, or equivalent) and are enforced through a quality assurance plan that is audited by the customer. The pass rate for UVAD-drilled holes in C/C composites at qualified production facilities (such as Bishen Precision) exceeds 98 percent, compared to pass rates below 10 percent for conventional carbide drilling.

Frequently Asked Questions ​

Why does conventional drilling fail on carbon-carbon composites? ​

Conventional carbide twist drilling fails on carbon-carbon (C/C) composites due to three interacting mechanisms: extreme tool wear, thermal damage, and delamination. The carbon fibres in C/C composites have a hardness of 2,000-3,000 HV (harder than tungsten carbide at 1,500-1,800 HV). When a carbide drill cuts through these fibres, the fibres abrade the cobalt binder that holds the tungsten carbide grains together. Once the binder is worn away, the carbide grains are pulled out individually, causing rapid flank wear on the drill. The flank wear rate for uncoated carbide drills in C/C composites is approximately 0.1-0.3 mm per millimetre of cutting length, which means the drill is effectively worn out within 1-5 mm of cumulative cutting. As the drill wears, the cutting forces increase because the worn cutting edge cannot shear the fibres efficiently. The increased forces generate higher cutting temperatures: temperatures at the drill tip can exceed 600 C, causing the carbon matrix to oxidise in air (oxidation begins at approximately 400 C) and the fibre-matrix bond to degrade (the bond strength decreases above 300 C). The thermal damage extends 0.5-2.0 mm from the hole edge, creating a zone of weakened material that may fail under the high-temperature, high-pressure gas loading during rocket motor firing. The high axial cutting forces from a worn drill (200-500 N for a 6 mm drill) cause delamination: at the hole entry, the drill pushes the surface fibres downward, separating the top plies from the underlying structure; at the hole exit, the drill pushes the last plies outward, creating a cone-shaped delamination that can extend 2-5 mm from the hole edge. The delamination factor (the ratio of the maximum delamination diameter to the hole diameter) for conventional drilling in C/C composites is typically 1.2-2.0, far exceeding the aerospace acceptance limit of 1.1. Even if a hole is successfully produced (the drill does not break), the delamination and thermal damage make the hole unsuitable for service.

How does ultrasonic vibration reduce cutting forces? ​

Ultrasonic vibration reduces cutting forces in composite drilling through three mechanisms: impact fracture, interrupted cutting, and friction reduction. The ultrasonic vibration system generates a high-frequency axial oscillation (typically 20-40 kHz) with an amplitude of 8-20 microns at the drill tip. This oscillation is superimposed on the rotation and feed motion of the drill, creating a cyclic variation in the instantaneous uncut chip thickness. (1) Impact fracture: The high-frequency axial oscillation causes the drill to impact the workpiece surface at the vibration frequency, delivering impact loads of 50-200 N at 40,000 impacts per second. These impacts create micro-fractures in the brittle carbon fibres ahead of the cutting edge, reducing the force required to fracture the fibres during the subsequent cutting cycle. The impact loading is particularly effective for brittle fibres (carbon, silicon carbide) that have low fracture toughness: the impact energy is concentrated in a small area, causing the fibre to fracture by cleavage rather than by the slower process of shear deformation. (2) Interrupted cutting: During each vibration cycle, the drill is in contact with the workpiece for only a portion of the cycle (typically 30-70 percent of the cycle, depending on the amplitude and the feed rate). During the non-contact portion of the cycle, the drill lifts off the workpiece, allowing the coolant to flow into the cutting zone and remove heat. The interrupted cutting also allows the chips to be evacuated from the flute, preventing chip packing that would increase the cutting forces. The intermittent contact reduces the average cutting force by 50-70 percent compared to continuous cutting at the same feed per revolution. (3) Friction reduction: The ultrasonic vibration creates a periodic separation between the drill flanks and the bore wall, reducing the sliding friction that contributes significantly to the total drilling torque. The friction reduction is most pronounced in the transverse direction: the average transverse force (which causes drill deflection and hole deviation) is reduced by 60-80 percent, improving the hole positional accuracy and verticality. The combination of these three mechanisms reduces the axial cutting force from 200-500 N (conventional) to 50-150 N (UVAD) for a 6 mm drill in C/C composite, which is below the threshold that causes delamination (typically 100-200 N for C/C composites, depending on the fibre orientation and layup).

What coolant is used for drilling C/C composites? ​

The coolant for drilling C/C composites must be chosen carefully because conventional water-based coolants and oil-based coolants can degrade the composite material and cause contamination. Water-based coolants are unsuitable because the C/C composite absorbs water through capillary action (the porosity of C/C composites is 5-15 percent). The absorbed water causes: (1) degradation of the fibre-matrix bond (water molecules penetrate the interface between the carbon fibres and the carbon matrix), (2) reduced mechanical properties (the absorbed water acts as a plasticiser, reducing the composite's stiffness and strength by 10-30 percent), and (3) outgassing during high-temperature service (the absorbed water vaporises at the operating temperature of the rocket nozzle, creating internal pressure that can cause delamination). Oil-based coolants are also unsuitable because they leave a residue on the composite surface that interferes with subsequent bonding, coating, or heat treatment operations, and the oil can be absorbed into the porosity of the composite. The recommended coolant for C/C composite drilling is vortex cold air: compressed air at 5-10 bar is passed through a vortex tube that separates the air into a hot stream (80-100 C) and a cold stream (-10 to -30 C). The cold air stream is directed at the drilling zone through a nozzle positioned 5-15 mm from the drill entry point. The vortex cold air provides: (1) cooling — the cold air removes heat from the cutting zone by forced convection, maintaining the drilling temperature below 120 C (well below the 400 C oxidation threshold and the 300 C fibre-matrix bond degradation threshold), (2) chip evacuation — the high-velocity air stream blows the chips away from the drilling zone, preventing chip packing in the drill flute, and (3) no contamination — the air leaves no residue on the composite surface and does not affect the material properties. For high-production applications or for drilling very deep holes where the cold air may not reach the full depth of the hole, cryogenic cooling with liquid nitrogen (LN2) is an alternative. The LN2 is delivered through a nozzle that directs the liquid nitrogen at the drill entry point; the LN2 evaporates on contact, providing intense cooling at -196 C. The nitrogen gas also displaces oxygen from the cutting zone, preventing oxidation of the carbon matrix. The disadvantage of cryogenic cooling is the cost of the LN2 (approximately $0.50-1.00 per kg) and the safety requirement for oxygen monitoring in the work area.

How is delamination detected and prevented? ​

Delamination in C/C composite drilling is detected through a combination of in-process monitoring and post-process inspection. In-process monitoring uses the spindle power signal to detect delamination events in real time: when delamination occurs, the axial cutting force drops suddenly (because the delaminated plies offer less resistance to the drill), which causes a corresponding drop in spindle power. Advanced monitoring systems compare the spindle power trace to a baseline model and trigger an alarm if the power drops below the expected range. The alarm alerts the operator to stop the drilling and inspect the hole before continuing with subsequent operations. Post-process inspection uses two methods: (1) Borescope inspection — a fibre-optic borescope (0.5-2.0 mm diameter) is inserted into the drilled hole, and the operator inspects the hole wall for visible delamination (separation between plies visible as a dark line or gap), fibre pullout (fibres protruding from the hole wall), and thermal damage (discolouration or glazing of the matrix). (2) Ultrasonic C-scan — an ultrasonic transducer (typically 10-50 MHz) scans the area around the hole in a raster pattern, and the reflected ultrasonic signal is analysed to detect sub-surface delamination that is not visible on the surface. The C-scan can detect delamination extending up to 5 mm from the hole edge, with a resolution of 0.1-0.5 mm depending on the frequency and the transducer design. Delamination prevention in UVAD is achieved through: (1) the reduced axial cutting force (50-150 N compared to 200-500 N for conventional drilling), which is below the delamination threshold for most C/C composite layups, (2) the use of a backup plate (a piece of the same composite material or a rigid polymer plate placed behind the workpiece at the drill exit) that supports the exit plies and prevents them from being pushed outward, and (3) feed rate reduction at the exit (reducing the feed by 50-70 percent during the last 1-2 mm of drilling) to minimise the breakout force. For threaded holes, form tapping (a tap that displaces the material rather than cutting it, creating a thread by cold-forming the C/C composite) produces threads with superior fibre integrity compared to cut tapping (a tap that cuts the threads, which tends to pull out fibres and create delamination). The form tap is made from PCD or CVD diamond-coated carbide and is designed with a smaller pitch diameter than a cut tap to account for the springback of the composite material after forming.

What tool materials are used for C/C composite drilling? ​

The tool materials used for C/C composite drilling are selected primarily for wear resistance against the abrasive carbon fibres, while maintaining adequate edge sharpness and fracture toughness. The most effective tool materials, ranked by performance, are: (1) CVD diamond-coated tungsten carbide — the most widely used production tool material for C/C and Cf/SiC drilling. The CVD coating (10-20 microns thick) provides hardness of 6,000-8,000 HV, which is sufficient to resist abrasion by the carbon fibres for 200-500 holes per tool. The tungsten carbide substrate provides the fracture toughness needed to prevent catastrophic tool failure. The coating must have good adhesion to the substrate: the substrate is pre-treated by chemical etching to create a micro-rough surface that provides mechanical interlocking for the diamond coating. (2) PCD-tipped carbide — used for micro-drilling (diameters below 1 mm) and for applications where the sharpest possible cutting edge is required. PCD (polycrystalline diamond) is a sintered composite of diamond grains (2-30 microns) in a cobalt binder. The PCD tip is brazed or sintered onto a carbide shank. PCD provides hardness of 7,000-8,000 HV and can be ground to a sharper edge than CVD diamond coating (edge radius 1-3 microns for PCD vs 3-10 microns for CVD). PCD-tipped drills achieve tool lives of 300-800 holes in C/C composites. (3) PCBN (polycrystalline cubic boron nitride) — used for drilling C/C composites that contain silicon carbide (Cf/SiC composites). The CBN grains have hardness of 4,000-5,000 HV and, unlike diamond, do not react chemically with silicon at the cutting temperature. Diamond tools (CVD or PCD) can react with silicon carbide at temperatures above 700 C (forming silicon carbide and graphite, which degrades the diamond), so PCBN is preferred for Cf/SiC composites if the cutting temperature may exceed this threshold. However, PCBN is less hard than diamond and has a shorter tool life (50-200 holes). (4) Uncoated tungsten carbide — only suitable for very short production runs (1-5 holes per tool) or for prototyping, where the low tool cost justifies the frequent tool changes. The drill geometry must be optimised for the specific C/C composite being drilled: the fibre orientation, the fibre volume fraction, and the matrix type (carbon vs silicon carbide) all affect the optimal geometry. For a unidirectional C/C composite with fibres oriented perpendicular to the drill axis, the point angle should be 90-100 degrees (to minimise the axial force that causes delamination) with a very sharp cutting edge (edge radius less than 3 microns). For a 2D laminate with alternating fibre orientations, the point angle should be 100-120 degrees with a wiper edge (a flat section of the cutting edge that smooths the cut fibres).

The information provided in this article is for general informational purposes only. Data and recommendations are based on published research and industry experience as of 2026.