The Engineer's Guide to Copper Tungsten Properties
Copper tungsten (CuW) is one of those engineering materials that sits in an unusual category. It's a composite, not an alloy, and its properties don't follow simple rules-of-mixtures from the constituent metals. For engineers specifying CuW for MV switchgear, vacuum interrupters, or other electrical contact applications, knowing how the properties behave matters for choosing the right grade and predicting service life.
This article walks through the major property categories, electrical, mechanical, thermal, and arc-related, with reference to GB/T 8320-2025 (the current Chinese national standard, effective 2026-05-01) and what each property actually means for a contact application.
What CuW Is, Structurally
CuW is a powder-metallurgy composite: a porous tungsten skeleton infiltrated with molten copper. The tungsten forms a continuous, interconnected network at typically 70–90% by weight; the copper fills the pore space between the tungsten particles. The result is a material where tungsten dominates the mechanical and thermal behavior, copper dominates the electrical behavior, and the two phases coexist with relatively little chemical interaction (tungsten and copper are mutually insoluble. They don't form a true alloy).
This structure is what gives CuW its useful combination of properties:
- The copper phase carries most of the current (because copper conducts ~30x better than tungsten)
- The tungsten skeleton provides mechanical strength and arc-erosion resistance (because tungsten is harder and has a much higher melting point)
- The composite is dense and gas-tight, which matters for vacuum interrupter applications where outgassing would compromise the vacuum
CuW grades are designated by tungsten content as a weight percentage: CuW70 = 70 wt% tungsten, 30 wt% copper. As tungsten content goes up, all the tungsten-dominated properties get stronger and the copper-dominated electrical properties get weaker. Standard grades for switchgear applications are CuW70, CuW75, CuW80, with CuW85 and CuW90 used in specialty applications.
Electrical Properties
Conductivity
Conductivity is the most important property for most contact applications. The contact has to carry continuous current without excessive heating between switching events.
Per GB/T 8320-2025 minimum requirements (Table 2 vacuum switch grades):
| Grade | Conductivity ≥ (% IACS) | Resistivity ≤ (µΩ·cm)¹ |
|---|---|---|
| CuW70 | 42 | 4.10 |
| CuW75 | 38 | 4.54 |
| CuW80 | 34 | 5.07 |
| CuW85 | 30 | 5.75 |
| CuW90 | 27 | 6.39 |
¹ Calculated from %IACS using ρ(µΩ·cm) = 1.7241/(%IACS/100)
For context, the % IACS reference is annealed copper at 1.7241 µΩ·cm = 100% IACS. So CuW70 at 42% IACS has roughly 2.4x the resistivity of pure copper. For a contact running at MV breaker continuous currents (typically hundreds to low thousands of amps), this means proportionally higher I²R heating at the contact than at the surrounding copper conductors.
The conductivity scales with copper content, but not perfectly linearly. The tungsten skeleton interferes with electron flow paths even though current preferentially follows the copper phase. Higher tungsten content (lower copper content) drops conductivity in a slightly accelerating curve.
Effect of Nickel Additions
Adding small amounts of nickel to CuW improves machinability and some interface behaviors but reduces conductivity. Per GB/T 8320-2025 Appendix A, the relationship is roughly:
- CuW70 with 0.01% Ni: ~43% IACS
- CuW70 with 1.00% Ni: ~29% IACS
That's a 30%+ relative drop in conductivity for 1% Ni addition. Ni-containing CuW is a specialty choice. Most standard switchgear CuW is Ni-free.
Mechanical Properties
Density
Density is the easiest property to measure and verify, float-test or weighed-volume measurement is direct. Per GB/T 8320-2025 vacuum switch grade minimums:
| Grade | Density ≥ (g/cm³) | Theoretical density (g/cm³)² | Relative density (%) |
|---|---|---|---|
| CuW70 | 13.80 | 14.50 | 95.2 |
| CuW75 | 14.50 | 15.21 | 95.3 |
| CuW80 | 15.15 | 15.99 | 94.7 |
| CuW85 | 15.90 | 16.85 | 94.4 |
| CuW90 | 16.75 | 17.80 | 94.1 |
² Theoretical based on rule-of-mixtures with W = 19.25 g/cm³, Cu = 8.94 g/cm³
The relative density (actual / theoretical) shows that production CuW typically runs at 94–96% of theoretical, meaning there's residual porosity at the 4–6% level even in well-produced material. For vacuum interrupter applications, this residual porosity needs to be closed-cell rather than connected pathways; otherwise outgassing would compromise the vacuum.
Hardness
Hardness (per GB/T 8320-2025, measured in HB, Brinell hardness):
| Grade | Hardness ≥ (HB) |
|---|---|
| CuW70 | 175 |
| CuW75 | 195 |
| CuW80 | 220 |
| CuW85 | 240 |
| CuW90 | 260 |
Hardness scales with tungsten content because tungsten is much harder than copper. For comparison, pure copper is around 40–90 HB depending on temper; pure tungsten is around 250+ HB. CuW80 at 220 HB is well within the practical machining range for carbide tooling.
Bending Strength
Bending strength (3-point bend test per GB/T 5586):
| Grade | Bending Strength ≥ (MPa) |
|---|---|
| CuW70 | 790 |
| CuW75 | 885 |
| CuW80 | 980 |
| CuW85 | 1080 |
| CuW90 | 1160 |
For context, structural steel is roughly 400–500 MPa yield. CuW grades at 800–1200 MPa bending strength are mechanically strong, well above typical service stresses in MV switchgear contact mounting and operation.
Bonded Joint Strength (Whole Contact)
When CuW is brazed to a copper or copper-alloy conductive end as a whole contact assembly (整体电触头 in GB/T 8320-2025), the joint strength requirements are:
- ≥185 MPa tensile strength for joints to Cu conductive end
- ≥226 MPa tensile strength for joints to CuCrZr or similar copper alloy ends
This is the joint, not the CuW itself. The joint is typically the weakest point in the assembly, well-designed brazed joints between CuW and copper achieve these minimums consistently.
Thermal Properties
Melting Behavior
CuW doesn't have a single melting point, the two phases melt independently. Copper melts at 1085°C, tungsten at 3422°C. Under arc conditions inside a vacuum interrupter, the local temperature can briefly exceed copper's melting point in a small surface region, vaporizing copper while the tungsten skeleton remains solid. This selective vaporization is part of why CuW handles arc duty better than pure copper, the tungsten skeleton maintains the contact's dimensional integrity even when local copper melts.
Thermal Conductivity
Thermal conductivity tracks roughly with electrical conductivity. CuW70 is around 180–200 W/(m·K); CuW80 is around 160–180 W/(m·K). For context, pure copper is around 400 W/(m·K). This matters for continuous-current heating dissipation. The contact has to shed heat into the surrounding conductors.
Thermal Expansion
Coefficient of thermal expansion (CTE) is between copper (~17 × 10⁻⁶ /K) and tungsten (~4.5 × 10⁻⁶ /K), weighted by phase content. For CuW70, CTE is roughly 11–12 × 10⁻⁶ /K; for CuW80, roughly 9–10. This matters for assemblies that span operating temperatures, at the brazed joint between CuW and copper, the CTE mismatch creates stress during thermal cycling.
Arc Behavior
This is where CuW earns its place in MV switchgear contacts. Pure copper exposed to vacuum arc erosion would not survive typical breaker service life, the copper vaporizes too easily. CuW's tungsten skeleton provides the structural backbone that survives the arc; the copper that vaporizes is selectively replaced (the surface chemistry essentially self-renews during repeated arcing) until the tungsten skeleton itself starts to erode.
Higher tungsten content gives slower arc erosion per operation. The trade-off is conductivity. For most MV applications, CuW70 handles typical breaker cycle counts adequately over decades of service. For higher-cycle or more severe arc duty applications, CuW75 or CuW80 extends service life. The exact advantage is application-dependent and best evaluated against your specific duty cycle.
CuW also has very low gas content requirements for vacuum applications. Per GB/T 8320-2025 vacuum switch grades, O ≤ 0.008% and N ≤ 0.002% by weight. Residual gas in the contact material would outgas during the vacuum interrupter's evacuation and sealing process, compromising the vacuum.
Other Properties That Matter
Machinability
CuW machines with carbide tooling. The tungsten skeleton makes it harder to cut than pure copper, slower feed rates, more conservative depth of cut, and more tool wear are typical. CuW80 is harder to machine than CuW70. Surface finish achievable is good, comparable to or better than pure copper machining.
Corrosion Resistance
CuW is reasonably corrosion-resistant in typical MV switchgear environments. The tungsten phase is essentially inert to atmospheric corrosion; the copper phase can patinate over decades but doesn't significantly affect electrical performance in sealed assemblies. For exposed applications, silver plating on non-arcing surfaces is standard.
Defect Tolerance
Per GB/T 8320-2025, CuW contacts should pass penetrant inspection (GB/T 24300) for 40.5 kV+ applications with no cracks, voids, or porosity. Vacuum switch grades are exempt from penetrant inspection in the standard, but other quality checks (density measurement, metallographic inspection) catch internal defects.
Summary
CuW's properties trade off in predictable directions as tungsten content increases: higher density, hardness, bending strength, and arc-erosion resistance; lower conductivity and slightly higher cost. For 12 / 24 / 40.5 kV MV switchgear contacts, the practical grade range is CuW70 through CuW80, with CuW70 the default for standard duty and higher grades for higher-cycle or higher-voltage-class applications.
Per GB/T 8320-2025, the standard minimums document specific performance floors that any factory selling CuW70 (or other grades) must meet. Actual lot properties typically run modestly above the minimums; certified material documentation per lot is available on request for applications requiring specific values.
For grade selection guidance specific to your application, see How to Choose CuW Grade. For the full CuW product range, see the Copper Tungsten Series.
Related Reading
- How to Choose CuW70 vs CuW75 vs CuW80: Grade selection guide
- Static vs Moving Arc Contact: How the contact pair works in a VI
- Copper Tungsten (CuW) Series: Full product range
- CuW Static Arc Contact: Standard arc contact